@instance macro should be able to take named type parameters, not just an ordered list

[kris-brown]

When one makes an instance of a theory, one must provide Julia types for each sort of the theory. The sorts are ordered, so one need not provide these via key-value pairs. However, when there are many such types, it becomes hard to remember the order when writing the code (and likewise it's very hard to understand what is going on when reading the code weeks later).

To solve this, @instance ought be able to parse pairs and implicitly generate the correctly-ordered list of types, throwing an error if there are missing or superfluous keys. For example:

@instance ThCategory{Ob=Int, Hom=Vector{Int}} [model::FinSetC] begin 
  ...
end

This is at odds with type constructor name overloading, but I'm pretty sure there are other places where actually doing that kind of overloading would lead to errors.