Expression unification gives TIRX a shared `Expr` surface, but its
visitor and mutator APIs still expose primitive-only signatures. That
mismatch prevents general expressions from flowing through the existing
traversal structure and leaves statement traversal with overlapping
customization hooks.
This refactor generalizes the existing `ExprFunctor`, `ExprVisitor`, and
`ExprMutator` signatures in place to accept and return `Expr`. Statement
visitors and mutators expose a single virtual `VisitExpr(const Expr&)`
hook, while primitive statement reconstruction uses a non-virtual
checked `VisitPrimExpr` helper so invalid narrowing fails at the
boundary. Public pre-order and post-order traversal entry points accept
general `Expr` roots.
The existing specialization, vtable, dispatch registration, and class
structure remain intact; the change adds no parallel functor, fallback
dispatcher, or alternate implementation path.
Tianqi Chen
d27ed727ce