`$crate` in macros is accepted in any identifier position

[petrochenkov]

E.g. this compiles on rustc 1.20.0-nightly (c9bb93576 2017-06-24)

macro_rules! m {
    () => {
        struct $crate {}
    }
}

m!();

fn main() {}

This is a regression from Rust 1.14.0.

$crate should be accepted only as a first segment in a path, so it would be reasonable to turn it from an "arbitrary identifier" into a "path segment keyword" like Self/self/super.