crypto: add scrypt() and scryptSync() methods

Scrypt is a password-based key derivation function that is designed to
be expensive both computationally and memory-wise in order to make
brute-force attacks unrewarding.

OpenSSL has had support for the scrypt algorithm since v1.1.0.  Add a
Node.js API modeled after `crypto.pbkdf2()` and `crypto.pbkdf2Sync()`.

Changes:

* Introduce helpers for copying buffers, collecting openssl errors, etc.

* Add new infrastructure for offloading crypto to a worker thread.

* Add a `AsyncWrap` JS class to simplify pbkdf2(), randomBytes() and
  scrypt().

Fixes: https://github.com/nodejs/node/issues/8417
PR-URL: https://github.com/nodejs/node/pull/20816


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>