Parser performance regression after disabling happy's --coerce option

According to @int-index via GHC-devs (https://mail.haskell.org/pipermail/ghc-devs/2019-May/017600.html):

This February I did some changes to the parser that require higher rank types support in ‘happy’. Unfortunately, as I discovered, happy’s --coerce option is severely broken in the presence of higher rank types, so I had to disable it. My benchmarks have shown a 10% slowdown from disabling --coerce (https://gist.github.com/int-index/38af0c5dd801088dc1de59eca4e55df4).

Alongside my changes I submitted a pull request to happy which fixes the issue (https://github.com/simonmar/happy/pull/134), in the hope that it would get merged, released, and I could re-enable --coerce in GHC ‘happy' configuration.

1. Merge https://github.com/simonmar/happy/pull/134
2. Release a new ‘happy’
3. (Optional) Specify in GHC’s build system that it builds only with the latest 'happy' release
4. Restore the --coerce option in GHC’s build system ‘happy’ configuration
5. Backport it to the ghc-8.8 branch

We also need to:

6. Update CI docker images