Rename VisitorMut::visit to VisitorMut::visit_mut

[niebayes]

There're cases we need to use Visitor::visit and VisitorMut::visit in the same context. Since they're of the same name, compiler would be confused.

For example:

        impl Visitor For NoopVisitor { .. }
        impl VisitorMut for NoopVisitorMut { .. }

        let mut visitor = NoopVisitor {};
        let _ = stmt.visit(&mut visitor);

        let mut visitor_mut = NoopVisitorMut {};
        let _ = stmt.visit(&mut visitor_mut);

Would raise the error:

the trait bound `NoopVisitorMut: sqlparser::ast::Visitor` is not satisfied
the trait `sqlparser::ast::Visitor` is implemented for `NoopVisitor`