`frame_support::storage`: Add `StorageStreamIter` (#12721)

* Save

* Add some test

* Yep

* Move to its own file

* More work

* More

* Finish implementation

* Start resolving comments and fixing bugs

* Fix all review comments

* Update frame/support/src/storage/stream_iter.rs

Co-authored-by: Koute <[email protected]>

* Update frame/support/src/storage/stream_iter.rs

Co-authored-by: Koute <[email protected]>

* Review feedback

* FMT

* Okay, let's initialize the values...

* Fix...

Co-authored-by: Koute <[email protected]>