Introspection (part 2) (#108)

Following on from #107, this PR updates the serialization layer to store
types separately from the serialized graph of objects.

The protobuf `Type` message carries the same information as a
`reflect.Type`, with a bit of extra metadata. Types are stored in an
array where the index is the type's unique ID. This allows us to encode
a graph of types (potentially with cycles) by referencing types by ID.

I had to remove the global type cache in favor of a local cache on each
`Serializer` instance. This allows us to produce a succint array that
encodes just the types that are relevant to a particular serialized
state, rather than having to work with a subgraph of all types (and a
sparse set of type IDs).