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/state.c

@@ -0,0 +1,49 @@
+/*
+* Copyright (c) 2021 Calvin Rose
+*
+* Permission is hereby granted, free of charge, to any person obtaining a copy
+* of this software and associated documentation files (the "Software"), to
+* deal in the Software without restriction, including without limitation the
+* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+* sell copies of the Software, and to permit persons to whom the Software is
+* furnished to do so, subject to the following conditions:
+*
+* The above copyright notice and this permission notice shall be included in
+* all copies or substantial portions of the Software.
+*
+* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+* IN THE SOFTWARE.
+*/
+
+#ifndef JANET_AMALG
+#include "features.h"
+#include <janet.h>
+#include "state.h"
+#endif
+
+JANET_THREAD_LOCAL JanetVM janet_vm;
+
+JanetVM *janet_vm_alloc(void) {
+    JanetVM *mem = janet_malloc(sizeof(JanetVM));
+    if (NULL == mem) {
+        JANET_OUT_OF_MEMORY;
+    }
+    return mem;
+}
+
+void janet_vm_free(JanetVM *vm) {
+    janet_free(vm);
+}
+
+void janet_vm_save(JanetVM *into) {
+    *into = janet_vm;
+}
+
+void janet_vm_load(JanetVM *from) {
+    janet_vm = *from;
+}