* Use 32-bit integers instead of pointers for compactness and
  serialization friendliness.
* Use a separate hash map for runtime and comptime capture scopes,
  avoiding the 1-bit union tag.
* Use a compact array representation instead of a tree of hash maps.
* Eliminate the only instance of ref-counting in the compiler, instead
  relying on garbage collection (not implemented yet but is the plan for
  almost all long-lived objects related to incremental compilation).

Because a code modification may need to access capture scope data, this
makes capture scope data long-lived state. My goal is to get incremental
compilation state serialization down to a single pwritev syscall, by
unifying the on-disk representation with the in-memory representation.
This commit eliminates the last remaining pointer field of
`Module.Decl`.