Add a "Split using NamedFieldPuns" code action

[isovector]

@kcsongor requested the ability to transform:

data MyRec = MyRec {a :: Int, b :: Text, c :: Char}
deconstruct :: MyRec -> Int
deconstruct m = _

into

data MyRec = MyRec {a :: Int, b :: Text, c :: Char}
deconstruct :: MyRec -> Int
deconstruct MyRec {a, b, c} = _

This PR adds this functionality under a new code action, available only for values of records. It correctly deals with GADTS and shadowing, opting to use a generated name in the latter case.