Optimize deserialize

[ethanhs]

Right now we classes in an enum to make things simple. This is actually rather slow, especially for single class stubs. We should remove this.

(testing shows this can be a 2x+ slow down!)

[ethanhs]

It seems this will be harder to do away with than I initially thought.

For now, it seems acceptable to be slower if you don't specify what type you want to deserialize to, and we can document dumps as having a signature Callable[[Union[str, bytes, bytearray], Type[T]], T].

To make deserialization faster in the enum case, we can implement deserialize for a struct with all attributes of all the structs we have. NB: use default attributes to allow for missing fields.

[ethanhs]

This was not as bad as I thought it would be! I still need to add support for the above signature, but that will be a new issue.