Invoking a type object with no parameters should *always* yield an instance of that type

[nikomatsakis]

I'm not sure what we do now for a call like uint8(), but I think it should yield 0. This basically means that we can say that for all type objects T, calling T() yields an instance of that type, calling T(exemplar) yields an instance of that type with data taken from exemplar.

[nikomatsakis]

The one form that is not applicable across all type objects is T(buffer, offset), though one can write T.Array(buffer, offset, 1) (or whatever) to create an array of length one.