implement function inlining (#2053)

- empty body
- single `AST_Return`
- single `AST_SimpleStatement`
- avoid `/*#__PURE__*/`

Miscellaneous
- enhance single-use function substitution

fixes #281

test/compress/evaluate.js

@@ -644,6 +644,7 @@ unsafe_prototype_function: {
 call_args: {
     options = {
         evaluate: true,
+        inline: true,
         reduce_vars: true,
         toplevel: true,
     }
@@ -665,6 +666,7 @@ call_args: {
 call_args_drop_param: {
     options = {
         evaluate: true,
+        inline: true,
         keep_fargs: false,
         reduce_vars: true,
         toplevel: true,