Add Storable instance for PlainRec.
Unroll the rLens' recursion over Elem values.

The manual unrolling improves GHC's ability to inline things,
completely eliminating the Implicit dictionary at runtime in some
cases.
Bump version; bump lens dependency.

The lens package deprecated the SimpleLens type alias in 3.8. Lens' is
the currently supported alias.
Change SimpleIso to Iso'.
Added a benchmark of Storable vinyl records.

The specific calculation is arbitrary, but it served to demonstrate a
performance issue in an earlier version of the vinyl package
surrounding rLens. When a record's type is known, traversing it can be
optimized to virtually eliminate the impact of Rec data constructors.
Fewer ghc-options for benchmark.
Cabal file cleanup.
Added Vinyl intro with doctests.

While the introduction document doesn't stress things very much, it at
least provides a smoke test of basic Vinyl functionality.
Define rLens in terms of the more general rLens'.

Provide rLens' to produce lenses of record fields of an arbitrary
functor type. rLens is now simply a specialization of rLens' from Rec
to PlainRec.

The new rLens' obviates the need for a helper function that had been
added to the Intro doctest.

Fix cabal test integration with the doctests. Previously, cabal test
did not actually run the tests because the main action in Intro.lhs
didn't do anything. Now it explicitly calls doctest.

Bump the package version.
Consistent use of PlainRec r over Rec r Identity
Documentation bits
Whitespace twiddling