syntax: Make `_` a reserved identifier
Why:
- Lexically `_` is an identifier.
- Internally it makes implementation of `use Trait as _;` (https://github.com/rust-lang/rust/issues/48216) and some other things cleaner.
- We prevent the externally observable effect of `_` being accepted by macros expecting `ident` by treating `_` specially in the `ident` matcher:
```rust
macro_rules! m {
($i: ident) => { let $i = 10; }
}
m!(_); // Still an error
```