message: Change `invoke_next_upcall` to return bool.
  Returning `YieldNoWaitReturn` was confusing because `invoke_next_upcall` is used by both yield-wait and yield-no-wait.
diff: @@ -23,7 +23,10 @@ pub(super) unsafe fn yield_no_wait(return_ptr: *mut libtock_platform::YieldNoWai
  }
  });
  - let upcall_ran = invoke_next_upcall();
  + let upcall_ran = match invoke_next_upcall() {
  + true => libtock_platform::YieldNoWaitReturn::Upcall,
  + false => libtock_platform::YieldNoWaitReturn::NoUpcall,
  + };
  unsafe {
  core::ptr::write(return_ptr, override_return.unwrap_or(upcall_ran));
  @@ -50,30 +53,30 @@ pub(super) fn yield_wait() {
  return;
  }
  - if invoke_next_upcall() == libtock_platform::YieldNoWaitReturn::NoUpcall {
  - // yield-wait was called but there is no upcall queued. In a real Tock
  - // system, this process would be put to sleep until an upcall was queued
  - // (e.g. by an interrupt). However, in this single-threaded test
  - // environment, there is no possibility a new upcall will be enqueued
  - // while we wait. Therefore we instead panic; tests and code under test
  - // should never call yield-wait with no upcall queued.
  - panic!("yield-wait called with no queued upcall");
  - }
  + // In a real Tock system, a process that calls yield-wait with no queued
  + // upcalls would be put to sleep until an upcall was queued (e.g. by an
  + // interrupt). However, in this single-threaded test environment, there is
  + // no possibility a new upcall will be enqueued while we wait. Panicing is
  + // friendlier than hanging, so we panic if there's no upcall.
  + assert!(
  + invoke_next_upcall(),
  + "yield-wait called with no queued upcall"
  + );
  }
  // Pops the next upcall off the kernel data's upcall queue and invokes it, or
  // does nothing if the upcall queue was entry. The return value indicates
  // whether an upcall was run. Panics if no kernel data is present.
  -fn invoke_next_upcall() -> libtock_platform::YieldNoWaitReturn {
  +fn invoke_next_upcall() -> bool {
  let option_queue_entry =
  with_kernel_data(|option_kernel_data| option_kernel_data.unwrap().upcall_queue.pop_front());
  match option_queue_entry {
  - None => libtock_platform::YieldNoWaitReturn::NoUpcall,
  + None => false,
  Some(queue_entry) => {
  unsafe {
  queue_entry.upcall.invoke(queue_entry.args);
  }
  - libtock_platform::YieldNoWaitReturn::Upcall
  + true
  }
  }
  }
  