The Visitor / VisitorMut traits had no hook for individual identifiers: Ident derived a no-op Visit, so identifiers could not be observed or rewritten during a traversal. This makes it impossible to, e.g., collect every identifier in a statement or rewrite identifiers in place.

This adds pre_visit_ident / post_visit_ident (default no-op) to both Visitor and VisitorMut, and gives Ident a manual Visit / VisitMut impl that invokes them.

Example — collect identifiers:

struct IdentVisitor { idents: Vec<String> }
impl Visitor for IdentVisitor {
    type Break = ();
    fn pre_visit_ident(&mut self, ident: &Ident) -> ControlFlow<()> {
        self.idents.push(ident.value.clone());
        ControlFlow::Continue(())
    }
}
// "SELECT a, b FROM t" -> ["a", "b", "t"]

Tests added in src/ast/visitor.rs for both the immutable and mutable hooks.