Rename VisitorMut::visit to VisitorMut::visit_mut

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:

``` Rust
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`
```

Search the repository for the VisitorMut trait and its visit method, then trace the implementations and call sites shown by those matches. Rename the method consistently and verify that Visitor and VisitorMut can be used in the same context without the reported trait-bound error; run the relevant existing Rust tests or checks.

Written by the indexing model from the issue text.