Gather the base16 code into its own file
Add a base16_literal conversion function
Optimize base16 decoding
Provide a complete hash ↔ base16 conversion API

* Add an encode_hash function
* Improve the decode_hash function signature
* Add a hash_literal function
Un-reverse the bitcoin_hash type in memory

With this change, the reversal only takes place in the encode_hash
and decode_hash functions.