First pass at implementing foreign classes.

Most of the pieces are there:

- You can declare a foreign class.
- It will call your C function to provide an allocator function.
- Whenever a foreign object is created, it calls the allocator.
- Foreign methods can access the foreign bytes of an object.
- Most of the runtime checking is in place for things like subclassing
  foreign classes.

There is still some loose ends to tie up:

- Finalizers are not called.
- Some of the error-handling could be better.
- The GC doesn't track how much memory a marked foreign object uses.

test/api/main.c

@@ -5,16 +5,23 @@
 #include "vm.h"
 #include "wren.h"
 
-#include "value.h"
+#include "foreign_class.h"
 #include "returns.h"
+#include "value.h"
 
-#define REGISTER_TEST(name, camelCase) \
-  if (strcmp(testName, #name) == 0) return camelCase##BindForeign(fullName)
+#define REGISTER_METHOD(name, camelCase) \
+  if (strcmp(testName, #name) == 0) return camelCase##BindMethod(fullName)
+
+#define REGISTER_CLASS(name, camelCase) \
+    if (strcmp(testName, #name) == 0) \
+    { \
+      camelCase##BindClass(className, &methods); \
+    }
 
 // The name of the currently executing API test.
 const char* testName;
 
-static WrenForeignMethodFn bindForeign(
+static WrenForeignMethodFn bindForeignMethod(
     WrenVM* vm, const char* module, const char* className,
     bool isStatic, const char* signature)
 {
@@ -29,8 +36,9 @@ static WrenForeignMethodFn bindForeign(
   strcat(fullName, ".");
   strcat(fullName, signature);
 
-  REGISTER_TEST(returns, returns);
-  REGISTER_TEST(value, value);
+  REGISTER_METHOD(foreign_class, foreignClass);
+  REGISTER_METHOD(returns, returns);
+  REGISTER_METHOD(value, value);
 
   fprintf(stderr,
       "Unknown foreign method '%s' for test '%s'\n", fullName, testName);
@@ -38,6 +46,18 @@ static WrenForeignMethodFn bindForeign(
   return NULL;
 }
 
+static WrenForeignClassMethods bindForeignClass(
+    WrenVM* vm, const char* module, const char* className)
+{
+  WrenForeignClassMethods methods = { NULL, NULL };
+  if (strcmp(module, "main") != 0) return methods;
+
+  REGISTER_CLASS(foreign_class, foreignClass);
+
+  return methods;
+}
+
+
 int main(int argc, const char* argv[])
 {
   if (argc != 2)
@@ -54,6 +74,7 @@ int main(int argc, const char* argv[])
   strcat(testPath, testName);
   strcat(testPath, ".wren");
 
-  runFile(bindForeign, testPath);
+  setForeignCallbacks(bindForeignMethod, bindForeignClass);
+  runFile(testPath);
   return 0;
 }