Improve error message when parsing Ident encounters keyword

[dtolnay]

Before:

error: expected identifier
 --> dev/main.rs:4:8
  |
4 |     fn async() {}
  |        ^^^^^

After:

error: expected identifier, found keyword `async`
 --> dev/main.rs:4:8
  |
4 |     fn async() {}
  |        ^^^^^

This lines up with how Rust's error reporting in macro_rules handles the same situation.

macro_rules! path {
    ($p:path) => {};
}

path!(async);

error: expected identifier, found keyword `async`
 --> src/lib.rs:5:7
  |
5 | path!(async);
  |       ^^^^^ expected identifier, found keyword