feat: subtree pairs iterator

[fominok]

This PR contains two changes:

Path compression

It alters how path compression works, previously collisions may occur for cases like [a, ab] vs [aa, b], also subtree keys shared the same prefix as parent trees which is harmful for iteration over subtree elements as there is no way to find out where to stop, now they're not.

New path compression algorithm (for subtree key):

1. Concat all path segments as before
2. Append number of segments
3. Append lengths of each segment
4. hash it

Subtree elements' keys use the same approach, but appending their key after hashing happened, this way their order is preserved and they're located sequentially in DB.

Subtree elements iterator

This allows for a given subtree to iterate over key-value pairs sorted by key, values are Element object