trafgen: lexer/parser: fix cpu() selection and whitespacing

It looks unintuitive when once needs to write cpu(0:3), so also allow
cpu(0-3). Then it's quite stupid to enforce whitespace after the double
colon before the curley braces open. So make this rather optional here.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
trafgen: parser: add possibility for negative numbers

The previous commit reduced it, so now we allow it within cX() containers
in order to negate a result, if someone needs that.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>