embassy-time: some driver_std fixes

[kaspar030]

I stumbled across a few oddities with the embassy-time std driver:

1. Both now() and schedule_wake() were locking inner. That made any use of now() within a waker cause a deadlock.

   => fixed by splitting inner into its fields queue and zero_instant, and using LazyLock for initialization.

2. The alarm_thread() iterates the timer queue while holding the queue lock, but outside a critical section. Now if:

   1. another thread holds a critical section and calls schedule_wake(), it is blocked on the queue lock
   2. alarm_thread() wakes a waker that in its wake() enters a critical section
   3. the two threads dead-lock

   => fixed by wrapping the queue iteration in a critical section.

3. Previously, schedule_wake() would just pass the Waker to the queue's schedule_wake() and signal the alarm thread. If two consecutive Wakers are scheduled, the first before now and the second after now, only the first would get woken (as soon as alarm_thread() processes the signal).
   This behaves differently than all the other time driver implementations, that all call queue.next_iteration() within schedule_wake() and thus execute already passed timeouts right away, causing two wakeups.

   While I believe consistent behavior alone justifies this change, it is also necessary for using the embassy-time machinery outside of tasks, because a Waker cannot re-schedule itself to a later timeout in its wake(), as calling schedule_wake() while the queue lock is held causes a deadlock. So a waker can only check if it should wake (through now() and its context). Through first setting it with at=0 (already passed), which wakes() once and then drops the waker from the queue, it can be set again with any at.

   => fixed by also calling queue.next_iteration() within schedule_wake().

[kaspar030]

I do see this is basically fixing three things, should I split this up? Or would that triple testing?

[Dirbaio]

we should avoid using critical_section in code targeting std. It's basically a global mutex, and using real mutexes allows for finer grained locking.