Bug 1842221 - Simplify ThinArc and friends. r=boris, a=dsmith

ThinArc is way more complex than it needs to be, and miri complains
about various things when using the selectors crate with it.

This doesn't fully fix miri, but it gets much closer. ThinArc becomes
just an alias for Arc<HeaderSlice<>>. This allows to simplify the
bindings to ArcSlice too, since now the existing Arc<> code can just be
used.

Differential Revision: https://phabricator.services.mozilla.com/D183011