Require minimum key size for OpenSSL keys

[Slamdunk]

Fixes #854

Few notes to read the PR effectively:

1. Hmac::minimumBytesLengthForKey has been renamed to Hmac::minimumBitsLengthForKey but it's not a BC-Break since it hasn't been released yet
2. Like in Require minimum key size for HMAC algorithm #835 (comment) main classes now require a safe minimum key length, Unsafe* classes behave like the previous one and have already been marked as @deprecated
3. Core classes have been copy-pasted, basically, in order to allow an easier just-delete-deprecated-classes in next major
4. TestCases have also been copy-pasted except for key length tests
5. openssl_sign error test ::signShouldRaiseAnExceptionWhenKeyIsInvalid used a short key to tests CannotSignPayload exception raise, but now that's blocked earlier and only UnsafeRsaTest covers those lines of code. In the next major I guess we'll need to just replace the if (! openssl_sign with an assert, maybe?
6. Doc has been updated too

MT fails because I've been a bit too wary:

 $details = openssl_pkey_get_details($key);
-        assert(array_key_exists('bits', $details) && is_int($details['bits']));
+        assert(array_key_exists('bits', $details) || is_int($details['bits']));

Honestly I don't have enough trust in PHP to delete this.

[lcobucci]

Thanks @Slamdunk! I'll review it ASAP (I've been dealing with so many things at work this week 😰)

[lcobucci]

Minor comments, I'll rebase your branch to handle them and we can get it merged.
Thanks for your help!

[lcobucci]

I think we're good for 4.2.0 🎉
I'll prepare the changelog and release it in the evening.

@Slamdunk thanks for all your help!