crypto/sha256: add WriteString and WriteByte method

This can reduce allocations when hashing a string or byte
rather than []byte.

For #38776

Change-Id: I1c6dd1bc018220784a05939e92b47558c0562110
Reviewed-on: https://go-review.googlesource.com/c/go/+/481478


Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>