We want to introduce the concept of missing elements to Simple-8b blocks.

The append(uint64_t) interface should be extended to also have skip(). The index is 0 based and should increment on every append, when skip is called it should skip ahead 1 from current index

When retrieving values from the decoder the associated index should also be returned. We can use std::pair<uint32_t, uint64_t> to represent this for now.

If append(12) is called followed by skip() and append(13) the output when decoding should be: (0, 12), (2, 13)

We want to encode the "index skip" by using the special bit pattern of all bits set to '1' in a Simple-8b bucket.