This patch only adds support to parts of `utils` and to `epoch`. Some
parts of `utils` had to be left out, since they rely on
`AtomicUsize::new` being `const` (which it is not in `loom`). Other
parts had to be left out due to the lack of `thread::Thread` in `loom`.
All the parts needed for `epoch` were successfully moved to loom.

For this initial patch, there are two loom tests, both in `epoch`. One
is a simple test of defer_destroy while a pin is held, and the other is
the Triber stack example. They both pass loom with
`LOOM_MAX_PREEMPTIONS=3` and `LOOM_MAX_PREEMPTIONS=2`. The latter tests
fewer possible interleavings, but completes in 13 minutes on my laptop
rather than ~2 hours. I have added loom testing of `epoch` to CI as
well.

Note that the uses of `UnsafeCell` in `utils` have not been moved to
`loom::cell::UnsafeCell`, as loom's `UnsafeCell` does not support `T:
?Sized`, which `AtomicCell` depends on.

Fixes crossbeam-rs#486.

Co-authored-by: Taiki Endo <te316e89@gmail.com>

Co-authored-by: Taiki Endo <te316e89@gmail.com>