[perf] `proc-macro` `Ident` should have method `as_str`

According to https://doc.rust-lang.org/proc_macro/struct.Ident.html

There are

- `pub fn new(string: &str, span: Span) -> Ident`

- `ident.to_string()`

Should provide below for performance

- `ident.as_str()`

Very common actual usage

```rs
ident_string = ident.to_string()

match ident_string.as_str() { ... }
```

Expect usage

```rs
match ident.as_str() { ... }
```
