[breaking] s/DataType/Type

[odow]

Closes #783

We should use Type instead of DataType.

The issue is that DataType is a concrete type of Type (confusing right) that causes the compiler problems (to quote Jeff "this sort of stuff drives the compiler crazy" and "if you're doing that, please stop").

julia> isconcretetype(DataType)
true

julia> subtypes(Type)
4-element Vector{Any}:
 Core.TypeofBottom
 DataType
 Union
 UnionAll

I tried a few other things, such as Tuple{Type{<:MOI.AbstractFunction},Type{<:MOI.AbstractSet}} but couldn't get them to work. It seems for a lot of these things, we want simpler type signatures with some run-time asserts to reduce our latency. These paths aren't hot and time-critical.

[blegat]

Looks good to me. I'm wondering whether I tried Tuple{Type,Type} at the time but if it works and the compiler does not like DataType then let's use that.