BUG FIX: Fix clear_unhandled_rejection() to work again.
BUG FIX: Pre-resolved promises no longer break race().
resolved() and rejected() are now implemented in XS rather than Perl.
Create all() and race() methods in Promise::XS::Promise.
finally() now creates its own internal result that has to be caught, or else an uncaught-rejection warning displays. The goal here is to match ES6’s behavior—as closely as possible, anyhow, given this library’s support for multi-part resolutions and rejections.
Rejections with no defined values now trigger a warning.