Add comparison operations for keys

[daurnimator]

Currenly to see if two key objects contain the same key I have to convert them to PEM and compare the strings.
Instead, it would be nice to have an __eq that uses EVP_PKEY_cmp_parameters.

[daurnimator]

EVP_PKEY_cmp_parameters doesn't work for all key types (notably RSA)
EVP_PKEY_cmp only checks the public keys match, not the private.

This doesn't seem to be possible without delving into each key type, so closing.

Useful test script:

#!/usr/bin/env lua

local pkey = require"openssl.pkey"

local a = pkey.new()
local b = pkey.new(a:toPEM("private"))
assert(a==b)
local c = pkey.new(a:toPEM("public"))
assert(a~=c)