Allow omitting type family signature

Syntactic issue.

I find myself writing

type family
  Not a where
  Not False = True
  Not True  = False

quite often, where the signature can be inferred it would be nice to be able to write ... to mean the same thing

type family
  Not False = True
  Not True  = False

I know Richard has a lot of things planned, I don't know how they mesh together

---

The desugaring is straightforward, for a type family with n-indexes, add a signature with n fresh type variables.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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