macros: Allow field path segments to be keywords

[svix-jplatte]

Motivation

Currently, a keyword like type fails compilation as (a path segment of) a field name, for no clear reason. Trying to use r#type instead leads to the r# being part of the field name, which is unhelpful¹.

Solution

Don't require the field path to match a macro_rules! expr, use repeated tt instead. I can't tell why this was ever required: The internal stringify macro was introduced in 55091c9#diff-315c02cd05738da173861537577d159833f70f79cfda8cd7cf1a0d7a28ace31b with an expr matcher without any explanation, and no tests are failing from making it match upstream's stringify! input format.

Special thanks to whoever implemented the unstable macro-backtrace feature in rustc, otherwise this would have been nigh impossible to track down!

¹ this can likely be fixed too by some sort of "unraw" macro that turns r#foo into foo, but that's a separate change not made in this PR

[hds]

Thanks for the tests! Could we please move them into the tracing crate, since that's where the macros that they're testing are defined.

[svix-jplatte]

Rebased to fix CI (hopefully).

[svix-jplatte]

I'm working on fixing the new CI failures ^^