Variant B promise refactoring (#33)

- Evaluate() method returns a Promise which settles when the module
  is ready.
- Move the [[ExecPromise]] internal slot to Source Text Module Record
  (eventual intended home: Circular Module Record).
- Avoid using constructs like AwaitWithReturn, BuiltinPromiseThen
  and even Await, and instead deal with Promise reactions directly
  for clarity.
- Specialize the spec-level Await to the particular case of continuing
  on to execute the module.
- Various small editorial cleanups/fixes.
- Editorial: Update parts of the spec to refer to [[ExecPromise]]
- Editorial: Upgrade "upon rejection" to ecmaspeak

Author: guybedford