Add a function for dealing with absurdity

Now, if a type is known to be uninhabited, `absurd` will immediately
find the proof of uninhabitedness and apply `FalseElim` to it. This can
inhabit any type.

Example:

    foo : Fin Z -> String
    foo f = absurd f
Merge pull request #746 from david-christiansen/absurdity

Add a function for dealing with absurdity