Use String rather than [Char] where possible

Try this in GHCi

Prelude> :t "foo"
"foo" :: [Char]

It would be better to say

"foo" :: String

Why don't we? Because of this in TysWiredIn

stringTy :: Type
stringTy = mkListTy charTy -- convenience only

That is, where GHC needs String is uses stringTy which is just [Char].

How to fix? Two ways:

1. Make String into a "wired-in type". That's not hard, but it increases the number of wired-in types, which is generally undesirable.
2. Make String into a "knonw-key name", and look it up in the type environment on the (few) occasions where we need stringTy. That's a little harder -- notably hsLitType would become monadic -- but not difficult.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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