Allow static string to be used as key in macros

[Thomasdezeeuw]

Currently it uses an indent, which doesn't allow for static strings. I would like to be able to do the following:

log::warn!(trace = log::as_display!("some trace id"); "message"); // Ok.
log::warn!("logging.googleapis.com/trace" = log::as_display!("some trace id"); "message"); // Doesn't compile.

It could be that this is very niche and complex to implement, in which case we can continue using our work around.

Related #436, /cc @KodrAus

[KodrAus]

I think this is something that would be pretty straightfoward using a tt-muncher, which we're already going to need fairly soon. We'd just need to support both ident and literal matches in our match arms. If you pass something other than a string through then we'll catch it attempting to convert that value into a Key.