* first impl of eth_getProof

* fixed docu

* added comments and refactored based on comments from holiman

* created structs

* handle errors correctly

* change Value to *hexutil.Big in order to have the same output as parity

* use ProofList as return type

Fixes #17581

This PR implements les.freeClientPool. It also adds a simulated clock
in common/mclock, which enables time-sensitive tests to run quickly
and still produce accurate results, and package common/prque which is
a generalised variant of prque that enables removing elements other
than the top one from the queue.

les.freeClientPool implements a client database that limits the
connection time of each client and manages accepting/rejecting
incoming connections and even kicking out some connected clients. The
pool calculates recent usage time for each known client (a value that
increases linearly when the client is connected and decreases
exponentially when not connected). Clients with lower recent usage are
preferred, unknown nodes have the highest priority. Already connected
nodes receive a small bias in their favor in order to avoid accepting
and instantly kicking out clients.

Note: the pool can use any string for client identification. Using
signature keys for that purpose would not make sense when being known
has a negative value for the client. Currently the LES protocol
manager uses IP addresses (without port address) to identify clients.

This package was meant to hold an improved 256 bit integer library, but
the effort was abandoned in 2015. AFAIK nothing ever used this package.
Time to say goodbye.

* common: all golint warnings removed

* common: fixups

Allow the --abi flag to be given - to indicate that it should read the
ABI information from standard input. It expects to read the solc output
with the --combined-json flag providing bin, abi, userdoc, devdoc, and
metadata, and works very similarly to the internal invocation of solc,
except it allows external invocation of solc.

This facilitates integration with more complex solc invocations, such
as invocations that require path remapping or --allow-paths tweaks.

Simple usage example:

    solc --combined-json bin,abi,userdoc,devdoc,metadata *.sol | abigen --abi -

This commit adds many comments and removes unused code.
It also removes the EmptyHash function, which had some uses
but was silly.

This avoids logging accept errors on shutdown and removes
a bit of duplication. It also fixes some goimports lint warnings.

* signer: introduce external signer command

* cmd/signer, rpc: Implement new signer. Add info about remote user to Context

* signer: refactored request/response, made use of urfave.cli

* cmd/signer: Use common flags

* cmd/signer: methods to validate calldata against abi

* cmd/signer: work on abi parser

* signer: add mutex around UI

* cmd/signer: add json 4byte directory, remove passwords from api

* cmd/signer: minor changes

* cmd/signer: Use ErrRequestDenied, enable lightkdf

* cmd/signer: implement tests

* cmd/signer: made possible for UI to modify tx parameters

* cmd/signer: refactors, removed channels in ui comms, added UI-api via stdin/out

* cmd/signer: Made lowercase json-definitions, added UI-signer test functionality

* cmd/signer: update documentation

* cmd/signer: fix bugs, improve abi detection, abi argument display

* cmd/signer: minor change in json format

* cmd/signer: rework json communication

* cmd/signer: implement mixcase addresses in API, fix json id bug

* cmd/signer: rename fromaccount, update pythonpoc with new json encoding format

* cmd/signer: make use of new abi interface

* signer: documentation

* signer/main: remove redundant  option

* signer: implement audit logging

* signer: create package 'signer', minor changes

* common: add 0x-prefix to mixcaseaddress in json marshalling + validation

* signer, rules, storage: implement rules + ephemeral storage for signer rules

* signer: implement OnApprovedTx, change signing response (API BREAKAGE)

* signer: refactoring + documentation

* signer/rules: implement dispatching to next handler

* signer: docs

* signer/rules: hide json-conversion from users, ensure context is cleaned

* signer: docs

* signer: implement validation rules, change signature of call_info

* signer: fix log flaw with string pointer

* signer: implement custom 4byte databsae that saves submitted signatures

* signer/storage: implement aes-gcm-backed credential storage

* accounts: implement json unmarshalling of url

* signer: fix listresponse, fix gas->uint64

* node: make http/ipc start methods public

* signer: add ipc capability+review concerns

* accounts: correct docstring

* signer: address review concerns

* rpc: go fmt -s

* signer: review concerns+ baptize Clef

* signer,node: move Start-functions to separate file

* signer: formatting

* common: delete StringToAddress, StringToHash

These functions are confusing because they don't parse hex, but use the
bytes of the string. This change removes them, replacing all uses of
StringToAddress(s) by BytesToAddress([]byte(s)).

* eth/filters: remove incorrect use of common.BytesToAddress

* core, vm, common: define constantinople fork, start implementation of shift instructions

* vm: more testcases

* vm: add tests for intpool erroneous intpool handling

* core, vm, common: fix constantinople review concerns

* vm: add string<->op definitions for new opcodes

This commit reduces database I/O by not writing every state trie to disk.

* common/fdlimit: Move fdlimit files to separate package

When go-ethereum is used as a library the calling program need to set
the FD limit.

This commit extract fdlimit files to a separate package so it can be
used outside of go-ethereum.

* common/fdlimit: Remove FdLimit from functions signature

* common/fdlimit: Rename fdlimit functions

Also unexport isHex, hasHexPrefix because IsHexAddress is the only caller.

Fixes #15550

* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter

Fixes #15004

This adds type and struct field context to error messages.
Instead of "hex string of odd length" users will now see "json: cannot
unmarshal hex string of odd length into Go struct field SendTxArgs.from
of type common.Address".

Byte padding function should return the given slice if the length is
smaller or equal rather than *only* smaller than.

This fix improves almost all EVM push operations.

* core: Fix for consensus test gasLimit > 2^63-1 https://github.com/ethereum/tests/blob/develop/BlockchainTests/bcInvalidHeaderTest.json#L238

* core: fix testcase for uncle gasUsage > gasLimit : https://github.com/ethereum/tests/blob/develop/BlockchainTests/EIP150/bcUncleHeaderValiditiy.json#L986

* math/big: rename TTM63m1 -> MaxBig63, + go fmt

* core: documentation