Add utitities for interrupting the event loop.

janet_loop1_interrupt makes the event loop compatible
with safe interruptions for custom scheduling. Does this by exposing
custom events on the event loop. A custom event schedules a function pointer
to run in a way that can interrupt
epoll_wait/poll/GetQueuedCompletionStatus.

src/core/vm.c

@@ -118,7 +118,7 @@
     if ((COND) && janet_vm.auto_suspend) { \
         janet_vm.auto_suspend = 0; \
         fiber->flags |= (JANET_FIBER_RESUME_NO_USEVAL | JANET_FIBER_RESUME_NO_SKIP); \
-        vm_return(JANET_SIGNAL_EVENT, janet_wrap_nil()); \
+        vm_return(JANET_SIGNAL_INTERRUPT, janet_wrap_nil()); \
     } \
 } while (0)
 #endif