fix self-assignment for OpenSSLHash::Digest, throw if context allocation fails, implement move support

Summary:
The copy assignment operator was not checking for self-assignment, which means the code was wrong when an object was being copied into itself.
Also, allocation failure in the constructor would be silently ignored and result in a segfault crash later. Throw std::runtime_error if OpenSSL context allocation fails.
Also, move constructor and move assignment operator were not implemented - they are now.

Reviewed By: yfeldblum

Differential Revision: D30879545

fbshipit-source-id: 8b06f6fe97912a03ec5480a3b7c69aebf3a7f2ca