Runtime storage

[joepetrowski]

The last month has seen an overhall of storage keys in Substrate. Add documentation about how storage keys are created (key elements + hasher), how to access storage from within the runtime, and how to query storage values over RPC.

Useful:

• https://github.com/paritytech/substrate/blob/c34e0641abe52249866b62fdb0c2aeed41903be4/frame/support/procedural/src/lib.rs
• Refactor away from opaque hashes paritytech/substrate#5226

[JoshOrndorff]

Related: https://substrate.dev/recipes/3-entrees/storage-api/storage-maps.html