Lenses that reduce a set of pointers into a smaller set of pointers.

Consider a use of, say, Enum.filter:

iex> [-2, -1, 0, 1, 2] |> Enum.filter(& &1 > 0)
[1, 2]

Functions in this module convert such a function into a lens that will (eventually) apply the same filter to already-selected elements of a container.

iex> lens = Lens.keys([:a, :c]) |> Lens.filter(& &1 > 0)
iex> container = %{a: 1, b: 9, c: -1}
iex> Deeply.get_all(container, lens)
[1]
iex> Deeply.update(container, lens, & -1111 * &1)
%{a: -1111, b: 9, c: -1}