[Serialization] Add a helper for serializing nodes in an object graph

The AST block in a compiled module represents an object graph, which
is essentially serialized in four steps:

- An entity (such as a Decl or Type) is encountered, given an ID, and
  added to a worklist.
- The next entity is popped from the worklist and its offset in the
  output stream is recorded.
- During the course of writing that entity, more entities will be
  referenced and added to the worklist.
- Once the entire worklist is drained, the offsets get written to a
  table in the Index block.

The implementation of this was duplicated for each kind of entity in
the AST block; this commit factors that out into a reusable helper.
No intended high-level functionality change, but the order in which
Decls and Types get emitted might change a little now that they're not
in the same queue.

Author: jrose-apple