…pper. r=botond,kats

In addition:
 - Move the fast hit tester to the rust side of the bindings.
 - Avoid blocking by requesting a hit tester early and only blocking if the request isn't delivered by the time of the first hit test query.

Differential Revision: https://phabricator.services.mozilla.com/D66994

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/6096a87c64ab2717f1b0c670589dad69ee219269