make stderr mutex recursive and lock it when panicking

[andrewrk]

This patch adds std.Thread.Mutex.Recursive which is a recursive mutex implementation (cc @kprotty for review).

Next, it changes the mutex used by std.debug.lockStdErr() to be a recursive mutex, and obtains the lock when panicking.

The previously used panic_mutex is no longer needed.

[andrewrk]

I realized that what I'm describing is simply making the stderr mutex a recursive mutex.

[kprotty]

Think this could be simplified:

lock():
  c = LOAD(&count, Unordered)
  if not (c > 0 and LOAD(&current_tid, Unorderd) == tid):
    mutex.lock() // make this failable for tryLock()
    c = LOAD(&count, Unordered)
    assert(c == 0) 
    STORE(&current_id, tid, Unordered)
  STORE(&count, c + 1, Unordered)

[kprotty]

TBH, an invalid_thread_id would be simplest:

lock():
  if LOAD(&current_tid, Unordered) != tid:
    mutex.lock()
    STORE(&current_tid, tid, Unordered)
  count += 1
  
unlock():
  count -= 1
  if count == 0:
    STORE(&current_tid, invalid_tid, Unordered)
    mutex.unlock()

[andrewrk]

Nice that last impl looks pretty good.