We cannot have empty Zig structs mapping to JS instances

[karlseguin]

An empty struct will share the same address as its sibling (1) which will cause an collision in the identity map.

(1) - This depends on Zig's non-guaranteed layout, so the collision might not be with its sibling, but rather some other [seemingly random] field.

[sjorsdonkers]

Lgtm
Could add an assert checking for zero sized fields

[karlseguin]

In _mapZigInstanceToJs I tried to add this:

if (isEmpty(ptr.child) == false) {
    // existing code
} else if (@TypeOf(js_obj_or_template) != v8.Object and @typeInfo(Receiver(ptr.child)) != .@"opaque" and @sizeOf(ptr.child) == 0) {
    // This else if was added
    @compileLog(@typeName(ptr.child));
} else {
    // existing comment
}

But I get a few false positives.I guess we could create a whitelist of types, but that gets pretty messy.