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/issue-1787.js

@@ -2,6 +2,7 @@ unary_prefix: {
     options = {
         evaluate: true,
         inline: true,
+        reduce_funcs: true,
         reduce_vars: true,
         unused: true,
     }