Feature request: update element key

[kaber2]

For SortedKeyList, it would be nice to have a more optimized way of updating an item's key than doing remove + add.

A few things that come to mind would be:

• determining whether there is no position change and in that case do nothing
• searching only maxes before/after the changed element for the new insertion point
• avoiding list shrinking and enlargement if the element will end up in the same sublist

[grantjenks]

These are good ideas and I’ve thought about it before. Can you propose an API as one or more methods to support?