Kind equalities update

[Richard Eisenberg]

Hi devs,

In a chat with Simon this morning about kind equalities, he expressed that you all might want to know about the plans for kind equalities in 8.0.

The wiki page both for user-facing changes and for implementation notes is here: https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1
It is my intent that all user-facing changes mentioned there will be available in 8.0. I am hard at work getting my branch ready to merge, and hope to do so mid-November.

Note: this is almost fully backward-compatible. Here are some areas where there are known incompatibilities:
- Template Haskell will have to be updated. But there's a raft of changes there, anyway.
- GHC will do a more careful job of checking for termination of instances regarding the use of kind variables. This may require new UndecidableInstances declarations. But the fact that these definitions (like `instance (C a, C b) => C (a b)` for a polykinded C) were accepted previously could be called a bug.

That's actually all I know of so far.

You can take kind equalities for a spin at github.com/goldfirere/ghc, on the "nokinds" branch. Note that this hasn't been merged with `master` since December of last year, so expect a little strange behavior compared with 7.10. These wrinkles will get smoothed out, of course.

Richard