add a way to map value at specific label

[safareli]

I needed something like this at one point let me know if you like this and will open PR.

mapLabel :: forall trash a b sym rIn rOut rBase
   . IsSymbol sym
  => RowCons sym a rBase rIn
  => RowCons sym b rBase rOut
  => SProxy sym
  -> (a -> b)
  -> Variant rIn
  -> Variant rOut
mapLabel p f r =
  -- NOTE: This coerce is safe as we know `rBase` is subset of `rOut`. `expand` could
  -- be  used instead, but than you need to provide `Union` instances when it's used.
  on p (inj p <<< f) (unsafeCoerce :: Variant rBase -> Variant rOut) r

[leestephenn]

I know of a few places that needed something like this.