96dfcdf3b7 Implement From for hash types (Noah)
Pull request description:
Reopening #1280 on the right branch + implemented `From` for references of types that were done in #1280.
ACKs for top commit:
Kixunil:
ACK 96dfcdf3b7
apoelstra:
ACK 96dfcdf3b7
Tree-SHA512: ad762032390f060b87cdd24033a5fc13a4c5297c55d7091ed89c5ca240be2f57998c0be084f40f9b04833920756b93a3ca4576a2ef944872354d1b3607734228