Type Class Derivation: Don't use ??? for TC[CNil]

Previous behaviour was unsound when the `TypeClass` instance always used both
arguments eagerly.

Consider an example derivation of an instance for `Int :+: String :+: CNil`.

Previously:
  TC.coproduct(intInstance,
    TC.coproduct(stringInstance, ???))

Type classes which "consume" data are unproblematic: There will never be a
value of type `CNil`, hence the `???` is never executed. On the other hand,
type classes which "produce" data, e.g. `Arbitrary`, would certainly want to
produce something for both cases.

The fix is conceptually simple. We introduce a new method on `TypeClass` called
`coproduct1` which simply maps an instance for type `T` into type `T :+: CNil`.

Now:
  TC.coproduct(intInstance, TC.coproduct1(stringInstance))

Users don't have to change anything, as there is a default implementation for
`coproduct1` (admittedly, not very efficient: it uses `project` internally).
override `coproduct1` in `Show` example to make output less confusing
Merge pull request #55 from larsrh/topic/coproduct1

Type Class Derivation: Don't use ??? for TC[CNil]