Ruby 2.2+ compatibility

[methodmissing]

References issue onelogin#8

Gist of the problem:

• Caused by conversion to the typed data system ( https://github.com/ruby/ruby/blob/trunk/doc/extension.rdoc#encapsulate-c-data-into-a-ruby-object ) for the openssl extension in ruby/ruby@163cb5b too
• Related issue was that the modules and classes for OpenSSL etc. was setup in the extension which can lead to namespace clobbering depending on usage context.

Gist of the fixes:

• Explicitly require openssl on extension init and let OpenSSL::Cipher and OpenSSL::OpenSSLError be references to the constants defined in ext/openssl instead.
• Fixed misuse of the Ruby convention of modules being prefixed with m and classes prefixed with c ( VALUE mOSSLCipher -> VALUE cOSSLCipher )
• Support the TypedData system with the secret sauce being the RTYPEDDATA_TYPE(obj) macro for parity for the typed data definition that the aead extension reads with the one that OpenSSL::Cipher allocates objects with.
• That way we're not setting ourselves up for potential fails on another upgrade with the proposed hacky fix in bytecaster@70a7405

@pallan @csfrancis @jnormore

[pallan]

My C is rusty but this looks okay to me

[csfrancis]

This is not being used.

[methodmissing]

Killed - thx.

[csfrancis]

LGTM 👍