Add ->* and yield

"wip"

Adds a basic version of generators. Tests courtesy of @alubbe.

What's (could be) missing : 
 - `yield from`
 - `function*`
 - tests for an edge case like `~~>*`
 - disable `class => ->*` (currently has an error, fixed on my local repo) (also for `!->*` if we go the hushed route)

Questions : 
 - Is this syntax okay ? (same for `function`)
 - Should these be hushed by default ? (they're here)