Convert ES5 style anonymous function expressions
to arrow functions if permissible by language semantics.
Note: `arrows` requires that the `ecma` compress option
is set to `6` or greater.
fixes#2150
- Use AST_Destructuring for lhf assignment patterns
- Use AST_DefaultAssign for default assignments
- Add more checks for lhs expressions
- Add lots of testing
- Cleanup ast (e.g. remove default property)
- Fix#1402 based on a patch from @kzc
- Refine spread allowance in array destructring pattern
- Add destructuring AST tree checker
