[crypto] add human readable format for hash
[crypto] add human readable format for hash

This will be useful when storing hashes in human readable format (i.e.,
secure storage using base64).

Closes: #2840
Approved by: JoshLind

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra