Improve code navigation in macros

[flodiebold]

Inspired by this forum post...

It'd be nice to provide some navigation features in macro definitions / expansions. E.g. if you navigate to the definition of println, you get

macro_rules! println {
    () => ($crate::print!("\n"));
    ($($arg:tt)*) => ({
        $crate::io::_print($crate::format_args_nl!($($arg)*));
    })
}

and can't click on _print, for example.

Here are various ideas, in order of how hard I expect them to be:

• we could add a heuristic to detect $crate paths and allow navigating to them in macro definitions.
• our "expand macro" functionality could be improved to show expansions inline and allow you to use navigation features in the expanded code. There's already #5949 about this, though I think it'd be cool to overlay the expansion inline.
• it might even be possible to keep some context when using "go to definition" on a macro call, which would allow us to interpret the macro definition in the context of the original call. I think IntelliJ does something similar with type parameters already, and I seem to remember talking about this with @matklad before, but I didn't find an existing issue about this.

[flodiebold]

(Labeling actionable for the first part, which I think should be pretty doable.)