Add secure_compare/2 BIF on the crypto module

[Bryan Frimin]

Hello everyone,

I propose to add `secure_compare/2` function on the `crypto` module.

Constant time memory comparison is a common mechanism for
applications/libraries dealing with crypto logic. It is easy to code
this function in Erlang[1], but the OpenSSL one is safe and fast.

It's why I suggest enriching the crypto module with this function. I've
already opened a pull request[2].

I tried as much as possible to follow the contribution guide. Don't
hesitate to tell me if I made a mistake somewhere.

[1] https://github.com/exograd/erl-pkcs5/blob/master/src/pkcs5.erl#L34
[2] https://github.com/erlang/otp/pull/4750

Regards,