`LocalSet` cleans up any tasks that have not yet completed when it is
dropped. Previously, this cleanup process required access to a
thread-local. If a `LocalSet` is stored in a thread-local itself, when
it is dropped, there is no guarantee the drop implementation will be
able to access the internal `LocalSet` thread-local as it may already
have been destroyed.

The internal `LocalSet` thread local is mostly used to avoid writing
unsafe code. All `LocalState` that cannot be moved across threads is
stored in the thread-local and accessed on demand.

This patch moves this local-only state into the `LocalSet`'s "shared"
struct. Because this struct *is* `Send`, the local-only state is stored
in `UnsafeCell` and callers must ensure to not touch it from other
threads.

A debug assertion is added to enforce this requirement in tests.

This reverts commit d8c7d27.