crypto: add randomFill and randomFillSync

crypto.randomFill and crypto.randomFillSync are similar to
crypto.randomBytes, but allow passing in a buffer as the first
argument. This allows us to reuse buffers to prevent having to
create a new one on every call.

PR-URL: https://github.com/nodejs/node/pull/10209


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>