Add janet_interpreter_interrupt for custom scheduling.

This would allow an embedder to suspend the current Janet fiber
via an external event like a signal, other thread, or really anything.
This is a useful primitive for custom schedulers that would call
janet_interpreter_interupt periodically (say, in an interval with SIG_ALRM),
do some work, and then use janet_continue on the janet_root_fiber, or
for embedding into other soft-realtime applications like a game. To say,
only allow about 5ms per frame of interpreter time.

src/core/state.c

@@ -28,6 +28,10 @@
 
 JANET_THREAD_LOCAL JanetVM janet_vm;
 
+JanetVM *janet_local_vm(void) {
+    return &janet_vm;
+}
+
 JanetVM *janet_vm_alloc(void) {
     JanetVM *mem = janet_malloc(sizeof(JanetVM));
     if (NULL == mem) {
@@ -47,3 +51,14 @@ void janet_vm_save(JanetVM *into) {
 void janet_vm_load(JanetVM *from) {
     janet_vm = *from;
 }
+
+/* Trigger suspension of the Janet vm by trying to
+ * exit the interpeter loop when convenient. You can optionally
+ * use NULL to interrupt the current VM when convenient */
+void janet_interpreter_interrupt(JanetVM *vm) {
+    if (NULL == vm) {
+        janet_vm.auto_suspend = 1;
+    } else {
+        vm->auto_suspend = 1;
+    }
+}