Improve panic messages in parse_quote! by embedding location

[nik-rev]

When syn is used inside of a proc-macro, we now embed the [file:line:column] information to panic messages. This makes it a lot easier to debug proc macros.

Turning this error message:

error: proc-macro derive panicked
 --> src/lib.rs:1:10
  |
1 | #[derive(Trait)]
  |          ^^^^^^
  |
  = help: message: expected string literal

Into an error like this:

error: proc-macro derive panicked
 --> src/lib.rs:1:10
  |
1 | #[derive(Trait)]
  |          ^^^^^^
  |
  = help: message: [/tmp/macr/src/lib.rs:9:10] expected string literal

Closes #1957

[nik-rev]

If you called this panic! instead (shorter), it would not compile on the msrv (1.68) because it would be ambiguous with a macro of the same name in the prelude. Later stable versions of Rust do not have this problem (e.g. 1.88)