Allow documentary type signatures

In some cases (e.g., lens) it's useful to document a binding with a more specific type signature than the true one, to show a useful special case. Unfortunately, this documentation can potentially be wrong or go out of date. I'd like to propose a pragma for this:

{-# Documentary #-} f :: T -> U

would ask the type checker to verify that f :: T -> U. Specifically, it would have the effect of creating and type checking, but then discarding, a hidden binding

f_doc :: T -> U
f_doc t = f t

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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