Disabling certain actions on constructor and relation

[FTRobbin]

With #461, we now have constructor and relation besides function. However, it is still allowed to do semantically dubious operations on them, and this leaks the underlying implementation details.

(sort foo)
(constructor bar (i64) foo)
(set (bar 0) (bar 1)) ; This currently creates then unions the two terms

(relation baz (i64 i64))
(let x (baz 0 1)) ; This creates the edge and binds `x` to unit

We want to forbid this kind of behavior.

A straightforward fix is to add those restrictions on the subtypes for those actions. The challenge is to make sure all cases are covered.