Task.Sleep(0) optimized. Refs: #156.
A task using Sleep(0) runs as fast as possible but without blocking the user interface.
Prime number calculation used as example for background task.
Robustness of trap handling after exhausting the stack space improved.
Coroutines.Init now traps if a coroutine cannot be created.
Otherwise it is hard to see where the problem comes from.

Signed-off-by: Josef Templ <josef.templ@gmail.com>