Generalized Child Tries (storage)

[4meta5]

Demonstrate usage of trie storage structure in the context of a minimal example

Rob: "What we're doing there is storing the root of another trie under a key in the main trie. Since keys are typically hashed in the main trie, it's a fairly convenient (although not the only) way to make sure that inserting many elements under a single mapping does not make your typical lookup path for unrelated elements longer."

• might reference specialized usage in crowdfund
• dependent on avoid key collision on child trie and proof on child trie paritytech/substrate#2209
• trie: add ChildTrie trait to simplify future child trie types addition paritytech/substrate#3421
• Conceptual: Substrate Storage polkadot-developers/substrate-developer-hub.github.io#255

cc @shawntabrizi