Verify the value of the equihash solution size field in block headers.
This field isn't stored in the BlockHeader struct, so we need to verify
it at parse time.
Part of #477.
Closes#158.
As discussed on the issue, this makes it possible to safely serialize
data into hashes, and encourages serializable data to make illegal
states unrepresentable.
Henry de Valence
teor
Jane Lusby
Deirdre Connolly