These changes fix two corner cases related to internal handling of types
in package rlp: The "tail" struct tag can only be applied to the last field.
The check for this was wrong and didn't allow for private fields after the
field with the tag. Unsupported types (e.g. structs containing int) which
implement either the Encoder or Decoder interface but not both 
couldn't be encoded/decoded.

Also fixes #19367

Prevents reallocation, improves performance

Issue found by @guidovranken

These functions allow destructuring of raw rlp-encoded bytes
without the overhead of reflection or copying.

The bug can cause crashes if Read is called after EOF has been returned.
No code performs such calls right now, but hitting the bug gets more
likely as rlp.EncodeToReader gets used in more places.

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

Decoding did not reject byte arrays of length one with a single element
b where 55 < b < 128. Such byte arrays must be rejected because
they must be encoded as the single byte b instead.

The list size checking overflowed if the size information
for a value was bigger than the list. This is resolved by
always performing the check before reading.

The rules have changed as follows:

* When decoding into pointers, empty values no longer produce
  a nil pointer. This can be overriden for struct fields using the
  struct tag "nil".
* When decoding into structs, the input list must contain an element
  for each field.

Input strings of length 1 containing a byte < 56 are non-minimal and
should be encoded as a single byte instead. Reject such strings.

All integers (including size information in type tags) need to be
encoded using the smallest possible encoding. This commit expands the
stricter validation introduced for *big.Int in commit 59597d23
to all integer types and size tags.

A single zero byte carries information and should not set the pointer
to nil. This is arguably a corner case. While here, fix the comment
to explain pointer reuse.

It is not safe to add anything to s.size.

This is a preliminary fix for #420 (SEC-18 RLP decoder unsafe
allocation). If a sane input limit is set on the rlp.Stream,
it should no longer be possible to cause huge []byte allocations.

The generic pointer decoder did not advance the input position
for empty values. This can lead to strange issues and even
infinite loops.

The encoder was missing a special case for one element strings whose
element is below 0x7f. Such strings must be encoded as a single byte
without a string header.

Über-convenience.

All other pointer types can handle nil just fine.