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
consolidate & enhance `unused` (#2439)...- defer declaration removal in `collapse_vars`
- account for `AST_SymbolFunarg` in deduplication
- private accounting for `collapse_vars`
- avoid issues with identity reference due to deep cloning
fixes #2437
safer `properties` transform (#2391)...`{ a: x, b: y }.a` => `[ x, y ][0]`
- `x` cannot be function containing `this`
`[ x, y, z ][1]` => `(x, z, y)`
- only if `z` is side-effect-free