Get rid of fiber for finalizers.

Instead, finalizers just get access to the foreign object's raw bytes.
This is deliberately limiting, since it discourages the user from
interacting with the VM in the middle of a GC.

src/include/wren.h

@@ -38,6 +38,12 @@ typedef void* (*WrenReallocateFn)(void* memory, size_t newSize);
 // A function callable from Wren code, but implemented in C.
 typedef void (*WrenForeignMethodFn)(WrenVM* vm);
 
+// A finalizer function for freeing resources owned by an instance of a foreign
+// class. Unlike most foreign methods, finalizers do not have access to the VM
+// and should not interact with it since it's in the middle of a garbage
+// collection.
+typedef void (*WrenFinalizerFn)(void* data);
+
 // Loads and returns the source code for the module [name].
 typedef char* (*WrenLoadModuleFn)(WrenVM* vm, const char* name);
 
@@ -64,7 +70,7 @@ typedef struct
   // foreign object's memory.
   //
   // This may be `NULL` if the foreign class does not need to finalize.
-  WrenForeignMethodFn finalize;
+  WrenFinalizerFn finalize;
 } WrenForeignClassMethods;
 
 // Returns a pair of pointers to the foreign methods used to allocate and