* core, crypto: various allocation savings regarding tx handling

* core: reduce allocs for gas price comparison

This change reduces the allocations needed for comparing different transactions to each other.
A call to `tx.GasPrice()` copies the gas price as it has to be safe against modifications and
also needs to be threadsafe. For comparing and ordering different transactions we don't need
these guarantees

* core: added tx.GasPriceIntCmp for comparison without allocation

adds a method to remove unneeded allocation in comparison to tx.gasPrice

* core/types: pool legacykeccak256 objects in rlpHash

rlpHash is by far the most used function in core that allocates a legacyKeccak256 object on each call.
Since it is so widely used it makes sense to add pooling here so we relieve the GC.
On my machine these changes result in > 100 MILLION less allocations and > 30 GB less allocated memory.

* reverted some changes

* reverted some changes

* trie: use crypto.KeccakState instead of replicating code

Co-authored-by: Martin Holst Swende <martin@swende.se>
# Conflicts:
#	core/tx_list.go
#	core/tx_pool.go
#	core/types/block.go
#	core/types/transaction.go
#	trie/committer.go
#	trie/hasher.go