Storable

Add `Storable` instances for `PlainRec`.

Included a simple benchmark to verify that a `Vector` of `PlainRec` values does not pick up any overhead from the `Rec` data constructors.

Added the introduction to Vinyl document as a doctest.

Provide a generalized `rLens` called `rLens'` that provides lenses for fields of an arbitrary functor type. `rLens` is now just a specialization of `rLens'` to `PlainRec`.