Introduce iterator-based load/store

[axic]

Currently it is based on arrays/vectors and byteswapping is performed afterwards. In many cases the inputs are iterators and could avoid byteswapping with reverse-iterating.

[axic]

Could also replace these with it: https://github.com/ethereum/evmone/blob/05e923fa2bfa004f4ed17978874baf18827332ed/lib/evmone/eof.hpp#L104-L120

[chfast]

This is only matter of API design. Can be done with generic Iterator type. However, Boost.Endian seems to only support unsigned char*.

[axic]

@chfast any chance to get some initial prototype of this?

[chfast]

Not right now.