Provide access to $crate in the macro impl

[carllerche]

I'm not sure this is possible, but it is a follow up from our talk on IRC.

I'm wondering if it would be possible for the decl macro to include $crate in the enum ProcMacroHack.

This would help dealing with cases where the caller renames the extern crate that has the macro def.

[dtolnay]

I tried this and it seems to work as I would expect on Rust 1.24 (but not earlier versions) edad180. I will try to see if we can provide an implementation that would work on older compilers.

[dtolnay]

Here is a sketchy modification that seems to make it work on any version of Rust. 1e15278

[dtolnay]

The sketchy way works for expression macros but not item macros. I would be okay making that change. @carllerche for your use case do you need this to work in item macros? I think that may be more complicated.

[dtolnay]

Closing in favor of rust-lang/rust#62325.