Move away from the `kobj_define` task declaration to use the new
`#[zephyr::thread]` to define these.  This allows for a more natural
declaration where the thread just looks like an attribute added to a
regular function declaration.

This also eliminates the static Mutex, as the Mutex now has a
constructor that avoids allocation (it is still put in an Arc, though).

Signed-off-by: David Brown <[email protected]>

To help with debugging, try to give created Zephyr threads a readable
name.  Currently, this is based off of the name of the function used in
the declaration of the thread.

Signed-off-by: David Brown <[email protected]>

Change this to be more general about the async executor used, instead of
the zephyr-specific workq-based one.

This doesn't change the sample itself, just the name.

Signed-off-by: David Brown <[email protected]>

The work-queue-based executor was an attempt to implement a Rust
executor that worked with Zephyr work queues.  Although it was possible
to make something that kind of worked, and did have the advantage of
being able to use a few Zephyr primitives, it suffers from several
problems:

- It isn't very efficient, basically combining the overhead of async
  scheduling, triggered work, and the Zephyr thread scheduler.
- It plays poorly with any other use of async.  Since it is useful to be
  able to use external crates for async utilities, this normally just
  results in undefined-behavior.

Now that we have another executor (built from Embassy), this code is
also mostly unneeded.

Signed-off-by: David Brown <[email protected]>

Convert away from the Zephyr-specific async to standard Rust async.
This uses the Embassy crates for time and synchronization.  All tasks
are spawned within a single executor that runs in the main thread.

Signed-off-by: David Brown <[email protected]>

Import the embassy crates, but not by path.  Also, remove the specific
time instance.  Although and specific application will likely want to
use a non-default tick rate, leaving at the 1us tick will allow the time
to be used (with a slight performance cost) with any time base.

Signed-off-by: David Brown <[email protected]>

The pin-weak provides a weak pointer that is compatible with `Pin<Arc>`.
Unfortunately, these are not compatible with the Arc that we use from
portable-atomic-utils.  Provide a simplified implementation of this
until the pin-weak is able to gain this functionality.

Signed-off-by: David Brown <[email protected]>

The current benchmarks are failing with a stack overflow on 64-bit
targets, so increase the size a bit.

Signed-off-by: David Brown <[email protected]>

Semaphores no longer use alloc in `new`.  Remove the incorrect
conditional based on allocation that remained.

Signed-off-by: David Brown <[email protected]>