Add bag type for storing constraints and type arguments

[desaikd]

We don't have a bag type. There are plenty of 3P bag/multiset types available, but I don't have a particular one in mind. Either way, we need it so that we have proper equality since the order of the constraints is unimportant.

If open content was modeled separately from the constraints, we could use a set instead of a bag since two completely identical constraints are redundant and functionally equivalent to having only one of them.

Originally posted by @popematt in #191 (comment)