implement compress option reduce_funcs (#2466)

- inline single-use function declarations as expressions when permissible
- depend on `reduce_vars`
- enabled by default
- disable for speed critical code

fixes #2464

test/compress/dead-code.js

@@ -292,6 +292,7 @@ global_timeout_and_interval_symbols: {
 issue_2233_2: {
     options = {
         pure_getters: "strict",
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         unsafe: true,
@@ -323,6 +324,7 @@ issue_2233_2: {
 issue_2233_3: {
     options = {
         pure_getters: "strict",
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         toplevel: true,