Remove C++ OpenSSL Bindings

[sjudson]

At the moment, the extcrypto.cc script [0] is used to provide two cryptographic functionalities: (i) generating RSA key material; and (ii) loading RSA key material stored in the .pem format.

As of Node 11, this functionality is available in the builtin crypto module, the former introduced in v10.12.0 [1] and the latter in v11.16.0[2]. As of May 1, 2021, Node v10 will no longer be supported [3]. As such, at that time the extcrypto module should be removed (as well as the dependency on nan) and the builtin functionalities used instead.

[0] https://github.com/sjudson/paseto.js/blob/master/extcrypto/extcrypto.cc
[1] https://nodejs.org/api/crypto.html#crypto_crypto_generatekeypair_type_options_callback
[2] https://nodejs.org/api/crypto.html#crypto_crypto_createprivatekey_key
[3] https://nodejs.org/en/about/releases/

[sjudson]

(created this issue mostly as a note to self, but if anyone has any comments/wants to open a PR they are more than welcome)