module: implement cross-Object-pool graph relations

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

examples/play-opus/main.zig

@@ -13,7 +13,8 @@ pub fn main() !void {
     const allocator = std.heap.c_allocator;
 
     // The set of Mach modules our application may use.
-    var mods = Modules.init(allocator);
+    var mods: Modules = undefined;
+    try mods.init(allocator);
     // TODO: enable mods.deinit(allocator); for allocator leak detection
     // defer mods.deinit(allocator);