Add an optional destructive combine API called "absorb".

* Implement absorb for LIST and remove custom window hack
* Add test for LIST segment trees and verify it doesn't trash anything
* Fix a small API wart in the list segment code.

PR feedback:

* Change header variable name
* Document new API in the README.

First refactor of segment trees for order-sensitive aggregates.

Handle order-sensitive aggregates segment trees.

Use an enum in AggregateInputData instead of creating a new API.

Generate AggregateCombineType enum strings...

Remove last references to the destructive combine API called "absorb".