Add janet_vm_save and janet_vm_load.

This lets a user multiplex multiple Janet VMs on a single
thread or process if they are willing to implement context switching
themselves.

src/core/wrap.c

@@ -163,7 +163,7 @@ Janet(janet_wrap_number)(double x) {
 void *janet_memalloc_empty(int32_t count) {
     int32_t i;
     void *mem = janet_malloc((size_t) count * sizeof(JanetKV));
-    janet_vm_next_collection += (size_t) count * sizeof(JanetKV);
+    janet_vm.next_collection += (size_t) count * sizeof(JanetKV);
     if (NULL == mem) {
         JANET_OUT_OF_MEMORY;
     }