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/properties.js

@@ -833,6 +833,7 @@ lhs_prop_2: {
         evaluate: true,
         inline: true,
         properties: true,
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         unused: true,
@@ -879,6 +880,7 @@ prop_side_effects_1: {
         evaluate: true,
         inline: true,
         properties: true,
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         toplevel: true,
@@ -915,6 +917,7 @@ prop_side_effects_2: {
         inline: true,
         passes: 2,
         properties: true,
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         toplevel: true,