Make Glob.literal() a public function

[orf]

Being able to detect if a given string is a glob expression or a literal string is very handy in some contexts.

As on example, we have a list of default AWS regions that we allow the user to specify a glob expression to match. However, the user may want to include another custom string as a region.

Detecting if the user has passed a glob expression or a literal string is currently not possible with this crate, which is a real shame. If literal() was public then the implementation above would be as simple as:

let globs: &[Glob] = ...;
let glob_set = GlobSet::builder().add(...);
let literal_regions = globs.iter().filter_map(|g| g.literal());
let matched_regions = DEFAULT_REGIONS.iter().filter(|region| glob_set.is_match(region));

return literal_regions.chain(matched_regions).collect();

I think that exposing the full AST or any specific internals is a bad idea, but being able to tell if a glob is a literal string is non-invasive (and simple!) enough to warrant exposing?