Add (dynamic) constants and types to editor

Adds support for editor to mutate global constants and types in a safe manner. This assumes no simultaneous calls to .edit will occur across any editors. Type ID deduplication is done by a linear scan for now, although we could of course switch this in the future. I've tested this and confirmed it does work properly for return modification in interprocedural SROA.

hercules_opt/src/dce.rs

@@ -25,7 +25,7 @@ pub fn dce(editor: &mut FunctionEditor) {
 
         // If a node on the worklist has 0 users, delete it. Add its uses onto
         // the worklist.
-        if editor.users(work).len() == 0 {
+        if editor.get_users(work).len() == 0 {
             let uses = get_uses(&editor.func().nodes[work.idx()]);
             let success = editor.edit(|edit| edit.delete_node(work));
             if success {