Non-terminal `ident` should not match keywords

[mzabaluev]

This example macro is currently not usable:

macro_rules! idents_followed_by_keyword {
    ($($slot:ident,)+ else) => ($(let $slot = 0;)+);
}

fn main() {
    idents_followed_by_keyword!(_a, else);
}

The reason is that the ident format specifier makes no distinction between keywords and bona fide identifiers, resulting in ambiguity when the matcher meets else. As keywords cannot be used in place of identifiers in transcription, it should break no existing macro if ident is changed to exclude keywords.

This will permit more useful, cleaner macro syntaxes.

[petrochenkov]

This works as expected and can't be changed now.
See rust-lang/rust#27899