Simplify generated generator code.

- Instead of storing things in the closure we store it on a context object.
- Remove the $G object
- Use a sentinel for termination
- Extract async function code into a runtime function to reduce code duplication.

Review URL: https://github.com/google/traceur-compiler/pull/689

Closes #689.
Fix the repl link in the README

Fixes issue #692

TBR=@arv

Review URL: https://github.com/google/traceur-compiler/pull/693

Closes #693.