Re-implement yield parsing improvements

[jugglinmike]

@caitp @rwaldron This is the latest in the series of rewrites (the prior patches along these lines were 6759ac9 and gh-3419).

Since the first commit intentionally regresses functionality, we should definitely use a merge commit to land this branch.

Commit messages:

[[FIX]] Improve parsing of `yield`

Re-implement the improvements reverted by the previous commit. Use
distinct logic to differentiate this new implementation, including the
following substantive improvements:

- Correct the reported location of warning W093 ("Did you mean to return
  a conditional instead of an assignment?")
- Permit the use of `yield` as an identifier outside of strict mode and
  generator function bodies
- Remove inaccurate restriction on `yield` within Mozilla generator
  functions
- Forward the context when parsing computed property names

[[CHORE]] Revert `yield` expression improvements

JSHint's parsing of `yield` expressions is derived from code first
contributed by GitHub user "usrbincc." That user could not be reached to
sign JSHint's Contributor License Agreement. Revert the user's
contribution to the application logic (preserving the corresponding
MIT-licensed test code) so that it may be re-written under the terms of
the Contributor License Agreement.

Revert the changes introduced by
88c862df3dba9e2cfa1e44d4be909099d8306c97

[coveralls]

Coverage remained the same at 100.0% when pulling 9f55656 on jugglinmike:revert-yield-6 into 43b6354 on jshint:v2.11.0.

[jugglinmike]

I've rebased this against the v2.11.0 branch so that we can trial it via a release candidate.

[jugglinmike]

@rwaldron I'm thinking that since no one is actively working on the v2.11.0 feature branch, I should rebase that on master before we merge this. What do you think?

[rwaldron]

@jugglinmike I trust your intuition 👍

[jugglinmike]

That's much appreciated, Rick. This is rebased and ready for review.

[rwaldron]

Nice work!!