buffer: add compare and equals methods

compare() works like String.localeCompare such that:

    Buffer.compare(a, b) === a.compare(b);

equals() does a native check to see if two buffers are equal.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
crypto: fix a couple of VC++ warnings

Signed-off-by: Fedor Indutny <fedor@indutny.com>
crypto: improve error messages

1) ThrowCryptoTypeErrors was not actually used for
   type-related errors. Removed it.
2) For AEAD modes, OpenSSL does not set any internal
   error information if Final does not complete suc-
   cessfully. Therefore, "TypeError:error:00000000:l
   ib(0):func(0):reason(0)" would be the error mess-
   age. Use a default message for these cases.

Signed-off-by: Fedor Indutny <fedor@indutny.com>