Add `When` type family

Does a type family like When from here belong somewhere in Data.Types.*

type
  When :: Bool -> Constraint -> Constraint
type family 
  When cond  cls where
  When True  cls = cls
  When False cls = ()

like when.

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett
Operating system
Architecture