When the expected ROM overhead of a function is bigger than the actual function,
it is better to provide the function as static inline function in the header.
The value of `queue->waiter` at the time the event was queued (with IRQs
disabled) was backed up to the stack-variable `waiter`. Thus, the test later on
for `waiter` checks if the queue was already claimed at the time the event
was queued. Therefore, there is no race.
Added `event_wait_multi()` that takes an array of event queues rather than
a single event queues. The queue with the lowest index will have the highest
priority.
