Fix pattern proc macro not working in macro_rules

[tremwil]

Currently, the following use of pattern! does not compile:

use pelite::pattern;
use pelite::pattern::Atom;

macro_rules! forward_pat {
    ($pat:literal) => {
        pattern!($pat)
    }
}

const PAT: &[Atom] = forward_pat("");

This is because macro_rules! capture expressions, like $pat, get wrapped in a TokenTree::Group with no delimiter to preserve the order of operations (see https://doc.rust-lang.org/beta/proc_macro/enum.Delimiter.html#variant.None).

This PR fixes this without adding dependencies on other parsing libraries by first checking if the input is a no-delimiter group, and if so, expanding before trying to match on a TokenTree::Literal.