FieldIterator

        fn FieldIterator(comptime T: type) type {
            return struct {
                len: usize,
                metadata: [*]const Metadata,
                items: [*]T,

                pub fn next(self: *@This()) ?*T {
                    while (self.len > 0) {
                        self.len -= 1;
                        const used = self.metadata[0].isUsed();
                        const item = &self.items[0];
                        self.metadata += 1;
                        self.items += 1;
                        if (used) {
                            return item;
                        }
                    }
                    return null;
                }
            };
        }