Maintain Syntax Compatibility with ES6 where possible and sensible

There are some features already implemented in Spider that have a similar but different syntax compared to ES6. These should be made consistent.

Examples I can think of:
- Splats. ES6 uses `...rest`, spider uses `rest...`
- `use` / require vs the ES6 Module syntax
- Class Syntax (already mentioned)
- Interchanged for-of should work with objects as well.
- List Comprehension:
- ES6 : `[for x in list x * 2]`
- Spider: `[x * 2 for x in list]`