Convert to parametric types.

~~DO NOT MERGE YET: waiting on https://github.com/JeffBezanson/FixedPoint.jl/pull/2.~~

This implements #42. Tedious but hopefully well worth it. I also added lots of conversion tests, and this caught at least one missing method.

I'm pretty excited about what this will allow in Images!

I think it should be covered by the outer constructor?