Use default with axiomatize module?

[sweirich]

Right now axiomatize module has the potential to introduce inconsistent axioms.

We could fix this by using default instead. I.e. instead of generating

Axiom mkFoo : Int -> Char -> Foo. 

we could generate

Definition mkFoo : Int -> Char -> Foo := GHC.Err.default. 

as long as we have a default value for type Foo.

If we go forward with this strategy, we should make default instance definition more automatic. But should do that already.