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/drop-unused.js

@@ -673,6 +673,7 @@ issue_1539: {
 vardef_value: {
     options = {
         keep_fnames: false,
+        reduce_funcs: true,
         reduce_vars: true,
         unused: true,
     }
@@ -734,6 +735,7 @@ assign_chain: {
 issue_1583: {
     options = {
         keep_fargs: true,
+        reduce_funcs: true,
         reduce_vars: true,
         unused: true,
     }
@@ -1080,6 +1082,7 @@ var_catch_toplevel: {
     options = {
         conditionals: true,
         negate_iife: true,
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         toplevel: true,
@@ -1114,6 +1117,7 @@ issue_2105_1: {
         collapse_vars: true,
         inline: true,
         passes: 3,
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         unused: true,
@@ -1156,6 +1160,7 @@ issue_2105_2: {
         passes: 3,
         properties: true,
         pure_getters: "strict",
+        reduce_funcs: true,
         reduce_vars: true,
         side_effects: true,
         unsafe: true,