Introspection (part 3) (#109)

Following on from #107 and #108, this PR updates the serialization layer
to store the set of functions along with associated type information.

Previously, the serialization layer would store a string identifier when
a function pointer was encountered, followed by additional objects if
the function was a closure. The deserialization layer relies on
information from the user/coroc (via `RegisterClosure`) as to which
functions are closures and what the layout of objects looks like in
memory. For an offline process to analyze durable coroutine state we
need to store this information alongside the state.

This PR updates the serialization to register functions/closures (and
types) as they're encountered in the object graph. Like types in #108,
functions are stored in an array where the index is used as a unique
identifier. When serializing functions/closures, the serialization layer
now stores the function ID (the index, not the name) as function
pointers are encountered, followed by closure objects. An offline
process now has the same information as the original program when it
comes to function types and closure memory layouts.