add a shallow traversal (biplate)

[aavogt]

I couldn't find an equivalent to biplate which just traverses the immediate children.

class Biplate s a where
  biplate :: Traversal' s a

instance HasTypesUsing (BiplateT s a) s s a a => Biplate s a where
  biplate f = typesUsing @(BiplateT s a) @a @s f

data BiplateT b c
type instance Children (BiplateT b c) a = ChildrenBiplate a b c
type family ChildrenBiplate (a :: Type) b c where
  ChildrenBiplate a a c = '[c]
  ChildrenBiplate a b c = '[]

-- example application to VisObject from vis / not-gloss which has Generic but not Data
buildBottom :: VisObject Double
buildBottom = Quad 0 (V3 220 0 0) (V3 220 220 0) (V3 0 220 0) white

-- | >>> buildTop
-- Quad (V3 0.0 0.0 280.0) (V3 220.0 0.0 280.0) (V3 220.0 220.0 280.0) (V3 0.0 220.0 280.0) (RGBA 1.0 1.0 1.0 1.0
-- here types doesn't work because a deep traversal of VisObject needs missing Generic instances
buildTop = buildBottom & biplate @_ @(V3 Double) . _3 .~ 280

Unusually, typesUsing puts the a parameter before the s. If we follow that convention instead, the expression could be biplate @(V3 Double).