Rename Q (TExp a) to Code a

I propose that we modify the typed template haskell API so that quoting an expression of type t results in a value of type Code t rather than Q (TExp a) and the splice construct requires an argument of type Code a to produce a value of type a.

The definition of Code is as follows.

newtype Code a = Code (Q (TExp a))

There are two good reasons to do this.

1. Q (TExp a) is ugly and leaks implementation details to the user. Using effects from the Q monad is unsafe and against the principle of staged programming.
2. Writing instances for Q (TExp a) is quite difficult. It's much easier when the newtype is defined but at the cost of constant wrapping and unwrapping.

Trac metadata

Trac field	Value
Version	8.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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