crypto/sha512: add WriteString and WriteByte method

crypto/sha512: add WriteString and WriteByte method

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

For #38776

Change-Id: I4926ae2749f6b167edbebb73d8f68763ffb2f0c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/483816
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Auto-Submit: Ian Lance Taylor <iant@google.com>