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
implement function inlining (#2053)...- empty body
- single `AST_Return`
- single `AST_SimpleStatement`
- avoid `/*#__PURE__*/`
Miscellaneous
- enhance single-use function substitution
fixes #281