Update runtime storage example (#365)

* WIP: build storage key with new hashing API

* Attempt to build up correct key

* Fmt

* Update to new hashing API

* WIP: precalculate prefix hashes

* Precalculate prefix hashes

* Fmt

* Remove redundant local hashing buffers

* Fix hasher name

* Add comment re: blake2_128_concat

* Apply suggestions from code review

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

* Explicit prelude imports

Co-authored-by: Hero Bird <robin.freyler@gmail.com>
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