c-storable: CStorable class

When binding to C via hsc2hs, you are expected to write your own Storable instances, and manually poke struct fields with any Storable type. Since all manner of non-C types are in Storable, it's easy to accidentally poke a Char instead of CChar, at which point you have memory corruption that may be very difficult to track down. One such bug was enough for me, so I created a parallel CStorable class that prevents that kind of bug by only including C types. If you are writing a new C binding, I recommend something higher-level than hsc2hs, such as c2hs, which I think should sidestep the problem entirely by verifying your types. But if you are already using hsc2hs and for whatever reason don't want subtle memory corruption bugs, you can import ForeignC instead of Foreign and Foreign.C, and see if you have any.