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/comparing.js

@@ -96,6 +96,7 @@ self_comparison_1: {
 self_comparison_2: {
     options = {
         comparisons: true,
+        reduce_funcs: true,
         reduce_vars: true,
         toplevel: true,
     }