Pre-RFC: Raw identifiers

[kennytm]

@scottmcm

A possible extension: this could allow both identifier#catch and keyword#catch (probably with shorter prefixes) as a way to also expose catch in Rust 1.0 (or multi-version) code.

Anything except r#… and br#… will break Macros 1.0.

macro_rules! x {
    ($a:ident # $b:ident) => {};
    ($c:ident) => { should error };
}
x!(identifier#catch);
x!(keyword#catch);