Explicitly say that *Table returned by MakeTable may not be
modified. Otherwise, this leads to very subtle bugs that may
or may not manifest themselves.

Same comment was made on package crc64, to keep the future
open to the caching tables that crc32 effectively does.

Fixes: #12487.

Change-Id: I2881bebb8b16f6f8564412172774c79c2593c6c1
Reviewed-on: https://go-review.googlesource.com/14258Reviewed-by: Ian Lance Taylor <iant@golang.org>