Bitset.splice

This is a support function for [gio.list_model.ListModel] handling, by mirroring the GlistModel::items-changed signal.

First, it "cuts" the values from position to removed from the bitset. That is, it removes all those values and shifts all larger values to the left by removed places.

Then, it "pastes" new room into the bitset by shifting all values larger than position by added spaces to the right. This frees up space that can then be filled.