consolidate & enhance unused (#2439)

- defer declaration removal in `collapse_vars`
- account for `AST_SymbolFunarg` in deduplication
- private accounting for `collapse_vars`
- avoid issues with identity reference due to deep cloning

fixes #2437

test/compress/issue-973.js

@@ -51,6 +51,7 @@ this_binding_collapse_vars: {
     options = {
         collapse_vars: true,
         toplevel: true,
+        unused: true,
     };
     input: {
         var c = a; c();