Marshaling objects

The first batch of marshaling objects are in the library now. They all live in the ffi module for now, but they will be useful in any situation where one might want to render values as bytes or retrieve values from bytes, so I may end up breaking them out into their own module. The current list is uint64, int64, uint32, int32, uint16, int16, uint8, int8, ascii, and utf8. A marshaling object is one that implements the to_bytes and from_bytes methods; these methods accept a value and return a sized sequence of bytes, and accept a byte buffer and return a value, respectively.