Derive traversals of a given type in a product.

Running example:

>>> :set -XTypeApplications
>>> :set -XDeriveGeneric
>>> :set -XScopedTypeVariables
>>> import GHC.Generics
>>> :m +Data.Generics.Internal.VL.Traversal
>>> :m +Data.Generics.Internal.VL.Lens
>>> :{
data WTree a w
  = Leaf a
  | Fork (WTree a w) (WTree a w)
  | WithWeight (WTree a w) w
  deriving (Generic, Show)
:}

The default traversal strategy types recurses into each node of the type using the Generic instance for the nodes. However, in general not all nodes will have a Generic instance. For example:

>>> data Opaque = Opaque String deriving Show
>>> myTree = WithWeight (Fork (Leaf (Opaque "foo")) (Leaf (Opaque "bar"))) False
>>> over (types @String) (++ "!") myTree
...
... | No instance for ‘Generic Opaque’
... |   arising from a generic traversal.
... |   Either derive the instance, or define a custom traversal using HasTypesCustom
...

In these cases, we can define a custom traversal strategy to override the generic behaviour for certain types. For a self-contained example, see the CustomChildren module in the tests directory.