* `analyzeContainer` now has an `outdated_decls` set as well as
   `deleted_decls`. Instead of queuing up outdated Decls for re-analysis
   right away, they are added to this new set. When processing the
   `deleted_decls` set, we remove deleted Decls from the
   `outdated_decls` set, to avoid deleted Decl pointers from being in
   the work_queue. Only after processing the deleted decls do we add
   analyze_decl work items to the queue.

 * Module.deletion_set is now an `AutoArrayHashMap` rather than `ArrayList`.
   `declareDeclDependency` will now remove a Decl from it as appropriate.
   When processing the `deletion_set` in `Compilation.performAllTheWork`,
   it now assumes all Decl in the set are to be deleted.

 * Fix crash when handling parse errors. Currently we unload the
   `ast.Tree` if any parse errors occur. Previously the code emitted a
   LazySrcLoc pointing to a token index, but then when we try to resolve
   the token index to a byte offset to create a compile error message,
   the  ast.Tree` would be unloaded. Now we use
   `LazySrcLoc.byte_abs` instead of `token_abs` so the error message can
   be created even with the `ast.Tree` unloaded.

Together, these changes solve a crash that happened with incremental
compilation when Decls were added and removed in some combinations.