Bug 1613440 added a checker to prevent a profusion of new threads.
Unfortunately, it operates directly on C++ source code, and there is no
equivalent for Rust code.
It _is_ possible to construct such an equivalent (see bug 1810242), but
it's nontrivial. For now, let's just prevent the use of dynamic thread
names, to minimize the profusion of uninterestingly-distinct thread
names in our `XPCOMSpinEventLoopStack` crash-report field.
Differential Revision: https://phabricator.services.mozilla.com/D166842
7f1075b129