The API guidelines say macro input should be evocative of the output.
`hash_newtype` didn't have this property.
This change makes it look exactly like the resulting struct, `$len`
parameter was removed since it's not needed, reversing is controlled
using an attribute. The macro is also better documented and ready to be
extended in the future.
The tagged SHA256 newtype is not yet modified because it has a more
complicated input parameters.
Closes#1648
06f1f027ab