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/screw-ie8.js

@@ -187,6 +187,7 @@ dont_screw_try_catch_undefined: {
 reduce_vars: {
     options = {
         evaluate: true,
+        reduce_funcs: true,
         reduce_vars: true,
         ie8: true,
         unused: true,