The idea here is to move towards a future where anonymous decls are
represented entirely by an `InternPool.Index`. This was needed to start
implementing `InternPool.getFuncDecl` which requires moving creation and
deletion of Decl objects into InternPool.

 * remove `Namespace.anon_decls`
 * remove the concept of cleaning up resources from anonymous decls,
   relying on InternPool instead.
 * move namespace and decl object allocation into InternPool