In many situations, we have a string on hand rather than a []byte and we want to write that string into a hash.Hash.
Converting to []byte to call hash.Hash.Write is unfortunate since it allocates.

I propose that all (or most) implementations of hash.Hash also implement io.StringWriter. This would not require any changes to the hash.Hash interface (which can't change for compatibility reasons), but would allow anyone calling io.WriteString(h, s) to automatically benefit from allocation-free hashing.