Make fat-arrow handle both rest args and default values for parameters.

- Extract includes that users of macros need to include as the macros expand to functions (only for autocurry so far).
- Adds a bunch of tests; class, functions, destructuring, pipeline.
- Remove rest.sjs macro, covered by 1-functions.sjs.
- Remove 1-default-args.sjs covered by 1-functions.sjs and 1-fat-arrow.sjs
- Add back in the rest for rest args.