Support TypeSystem <-> SerializableTypeSystem conversion

Summary:
Adds round-trip conversion between `SerializableTypeSystem` and `TypeSystem` instances. (specifically `IndexedTypeSystem`).

- Construction uses `TryFrom<SerializableTypeSystem>`
- Any `TypeSystem` has a default serialization implementating using URI-based export
- Specific `TypeSystem` implementations can override this implementation where appropriate.

Reviewed By: praihan

Differential Revision: D96979371

fbshipit-source-id: fa193344bb5c6fde4b56c4699ff206867ab833d2

Author: sadroeck