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`
```

在仓库中搜索 VisitorMut trait 及其 visit 方法，然后跟踪这些匹配项所显示的实现和调用点。统一重命名该方法，并验证 Visitor 和 VisitorMut 能否在同一上下文中使用而不会出现所报告的 trait-bound 错误；运行相关的现有 Rust 测试或检查。

由索引模型根据 Issue 内容生成。