clean internal nodes on remove

[nicolaskagami]

What does this PR add?

• Cleans empty internal nodes on remove

closes #9

Details

• An argument could be made that it's not worth it to proactively clean these entries, as the original poptrie does. However, this is more in line with the other design choices.
• Added a regression test for when checking a removed entry from a parent that would've had the same stride id after removing a child node.