Improve panic messages when in `proc_macro`

[nik-rev]

It can be hard to figure out which parse_quote! invocation failed because -Zmacro-backtrace is unstable + disabled by default.

What do you think about adding [file:line:column] information to panic!() messages inside of parse_quote! and parse_quote_spanned!, and others places where panic!() is used?

We would define a panic! macro as follows:

macro_rules! panic {
    ($message:literal $($tt:tt)*) => {{
        #[cfg(feature = "proc-macro")]
        let is_available = proc_macro::is_available();
        if is_available {
            let location = core::panic::Location::caller();
            let file = location.file();
            let line = location.line();
            let column = location.column();
            ::core::panic!(concat!("[{}:{}:{}] ", $message), file, line, column $($tt)*)
        } else {
            ::core::panic!($message $($tt)*)
        }
    }};
}

Then use it everywhere in the crate.

This will make it easier to debug proc-macros, by turning an error message like this:

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

Into the following:

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