Generate (optional) instances for generated types

[edsko]

We should automatically derive things like Eq, Ord, etc. for the types we generate (structs, enums, typedefs, ..). However, as part of #173 we discussed the importance of using Haskell newtype rather than type to translate C typedefs: we should be able to provide different instances. This implies it must also be possible to prevent hs-bindgen from generating instances for specific types (note that rust bindgen also has features for preventing the derivation of certain classes, see #21).

[edsko]

The set of instances we should derive automatically will probably depend on the type (Num for example makes sense only for numeric types).

[edsko]

Related also #308.