Add move_index to change the position of an entry

[cuviper]

This moves the position of a key-value pair from one index to another by
shifting all other pairs in-between, making this an O(n) operation.

This could be used as a building-block for other operations, like #173
which wants to insert at a particular index. You can insert_full to
insert it somewhere, then choose whether to move_index depending on
whether you want to also change pre-existing entries.

[cuviper]

I'm not entirely committed to the name move_index. It's somewhat similar to swap_indices, but I chose singular index because the movement of the one from index is the main point, while all the other shifted indices are a side-effect. Semantically, this is equivalent to let value = vec.remove(from); vec.insert(to, value);, or a slice rotation as this implementation actually does in its entries. Maybe shift or rotate would be better in the name.

[cuviper]

I just noticed that #83 had a similar method called reorder_entry_index.

[praveenperera]

I would love to get access to this function. Still thinking about merging it in?

[cuviper]

Sure -- do you have any thoughts about the name?

[praveenperera]

I think move_index works best.

Reads really nicely as move index from x to y

Or move_entry_index

[praveenperera]

Ignore my last comment if you saw it (now deleted). This works great, I tired it out on my fork. This is exactly what I needed to make IndexSet and IndexMap work with my data structure.