Add an API for storing handles to values outside of the VM.

It's pretty bare bones now, but it lets you get a reference to an object
from a foreign call and then return it later.

test/api/get_value/get_value.c

@@ -0,0 +1,24 @@
+#include <string.h>
+
+#include "get_value.h"
+
+static WrenValue* value;
+
+static void setValue(WrenVM* vm)
+{
+  value = wrenGetArgumentValue(vm, 1);
+}
+
+static void getValue(WrenVM* vm)
+{
+  wrenReturnValue(vm, value);
+  wrenReleaseValue(vm, value);
+}
+
+WrenForeignMethodFn getValueBindForeign(const char* signature)
+{
+  if (strcmp(signature, "static Api.value=(_)") == 0) return setValue;
+  if (strcmp(signature, "static Api.value") == 0) return getValue;
+
+  return NULL;
+}

test/api/get_value/get_value.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn getValueBindForeign(const char* signature);

test/api/get_value/get_value.wren

@@ -0,0 +1,14 @@
+class Api {
+  foreign static value=(value)
+  foreign static value
+}
+
+Api.value = ["list", "of", "strings"]
+
+// Do some stuff to trigger a GC (at least when GC stress testing enabled).
+var s = "string"
+for (i in 1...10) {
+  s = s + " more"
+}
+
+IO.print(Api.value) // expect: [list, of, strings]