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

README.md

@@ -689,6 +689,11 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
   Specify `"strict"` to treat `foo.bar` as side-effect-free only when
   `foo` is certain to not throw, i.e. not `null` or `undefined`.
 
+- `reduce_funcs` (default: `true`) -- Allows single-use functions
+  to be inlined as function expressions when permissible.
+  Enabled by default.  Option depends on `reduce_vars` being enabled.
+  For speed critical code this option should be disabled.
+
 - `reduce_vars` (default: `true`) -- Improve optimization on variables assigned with and
   used as constant values.