Separate HIR Assignments and Constraints

[DavePearce]

At the moment, HIR assignments and constraints reuse the same underlying primitives as for MIR and AIR. However, there are some problems here:

• defconstraint supports both domains, guards and perspectives.
• defpermutation is a combination assignment and constraint.

The tricky question is whether we want hir.Schema to implement Schema or not. The issue being that Schema splits out constraints and assignments, whereas at the hir level they are (in some cases) merged together.

The other question is whether we actually need to do this. Its not clear that its really necessary at this stage. What is the benefit? Basically when we debug at the hir level we have something that looks like the original corset.