Interpret a term in an environment.

Note that for PrimalSpan term, the results of this function land in target y and not in PrimalOf target y. To make it sound nevertheless, we maintain an invariant that a value of interpretation of a term with PrimalSpan has zero dual part and of a term with DualSpan has zero primal part. The invariants holds by the properties of instances of Ops (see especially the ADVal instance, which zeroes dual part of many ops) and structural induction on Ast, inspecting spans of constructors. This promotion from PrimalOf target y to target y coincides with how most operations that in Ast have PrimalSpan, don't have PrimalOf (but have full target instead) in their method signatures in Ops, for user convenience. See, e.g., AstConcreteS vs tsconcrete and AstFloorS vs tsfloor.