Auto merge of #43076 - Zoxc:gen, r=arielb1

Generator support

This adds experimental support for generators intended to land once rust-lang/rfcs#2033 is approved.

This is not yet ready to be merged. Things to do:
- [x] Make closure arguments on generators an error
- [x] Spot FIXMEs
- [x] Pass make tidy
- [x] Write tests
- [x] Document the current syntax and semantics for generators somewhere
- [x] Use proper error message numbers
- [x] ~~Make the implicit argument type default to `()`~~

Author: bors