Complete fixint support

[jheretic]

We support the msgpack 'fixint' types, where small integers can be contained within a reserve range of the 'type' tag in order to pass small integers around with minimal overhead. However, we have very limited use for these numbers, as we don't have any useful functions to actually do things with them, like math. We should implement at least a basic set of accessor and math functions for these types so that we can utilize them.