Add method to get full match from Captures

[tmccombs]

Describe your feature request

Add a method to regex::Captures that is equvalent to (or even just this implementation):

#[inline]
fn full_match(&self) -> Match<'h'> {
    self.get(0).unwrap()
}

Getting the full match is a very common operation, and although the documetation for Regex::get says that getting the result at index 0 will always return Some, that property isn't reflected in the type system. And adding a method like this would expose that guarantee through the type of the public API.

[BurntSushi]

I think this is probably a good idea. I added an analogous method (among others) to the lower level regex-automata::util::captures::Captures type.

I'm not sure full_match is the right name though. Hmmm. I'm not a huge fan of get_match either in this context. Probably match would be ideally, except for the obvious drawback that match is a Rust keyword.

[tmccombs]

Yeah, I'm not sure full_match is a great name either, but I couldn't think of a better one.