Possible batch update of values

[cheme]

When updating multiple value, calling 'insert' multiple time on a 'triedbmut' does not look as efficient as doing it in a single pass.
With a batch update, we can avoid putting the whole trie changes in memory.
This kind of batch update is only possible if values to change are sorted.
It does not require caching node values like current triedbmut does (just a stack of node).
It is basically just moving the sort done by unordered triemut insertion to a previous processing.

In substrate the trie is build by 'storage_root' call https://github.com/paritytech/substrate/blob/2ac4dd8a7e116c40a80bf443d942da7163cad8ca/primitives/state-machine/src/trie_backend.rs#L153 , relying on triedbmut multiple calls to 'insert':
https://github.com/paritytech/substrate/blob/2ac4dd8a7e116c40a80bf443d942da7163cad8ca/primitives/trie/src/lib.rs#L133 .
If keys in substrate change overlay were to be stored in a sorted manner at an additional cost (an hypothesis for paritytech/substrate#4185), this cost could probably be leveraged by having a batch insertion in trie afterward (it needs a bench to evaluate what kind of improvement we can get from a batch insertion implementation, it should not be much, but neither should be the loss from sorting key in substrate change overlay).