Refactor OverlayedChanges (#5989)

* Hide internal structure of OverlayChanges

* Fix tests for OverlayChanges refactor

* Do not clone pending changes

Discarding prospective changes should be equivalent as a state machine
is not to be called with peding changes.

This will be replaced by a storage transaction that is rolled back before
executing the call the second time removing this constraint.

* Doc fixes

* Remove overlong line

* Revert "Do not clone pending changes"

This reverts commit 4799491f.

* Deduplicate chield tries returned from child_infos()

* Remove redundant type annotation

* Avoid changing the storage root in tests

* Preserve extrinsic indices in trie build test

* Swap order of comitted and prospective in fn child_infos

This is only for consistency and does not impact the result.

* Rename set_pending to replace_pending for clearity