Simon Peyton Jones authored Jun 05, 2019 and Marge Bot committed Jun 19, 2019

Partial type sigs had grown hair.  tcHsParialSigType was
doing lots of unnecessary work, and tcInstSig was cloning it
unnecessarily -- and the result didn't even work: #16728.

This patch cleans it all up, described by TcHsType
  Note [Checking parital type signatures]

I basically just deleted code... but very carefully!

Some refactoring along the way

* Distinguish more explicintly between "anonymous" wildcards "_"
  and "named" wildcards "_a".  I changed the names of a number
  of functions to make this distinction much more apparent.

The patch also revealed that the code in `TcExpr`
that implements the special typing rule for `($)` was wrong.
It called `getRuntimeRep` in a situation where where was no
particular reason to suppose that the thing had kind `TYPE r`.

This caused a crash in typecheck/should_run/T10846.

The fix was easy, and actually simplifies the code in `TcExpr`
quite a bit.  Hooray.