Use Defined for Instances instead of Qed

[jbaum98]

Using Qed for typeclass instances creates opaque identifiers that can't be evaluated during computation.

Compute 1 ?= 1.
     = (let (eqb, _) := nat_EqDec in eqb) 1 1

If we use Defined instead, the definitions are transparent and can be evaluated as expected.

Compute 1 ?= 1.
     = true

[andrew-appel]

Closing this pull request without merging, because it would be better submitted to the fcf repo, https://github.com/adampetcher/fcf