StorageNMap follow up improvments

[gui1117]

it seems follow up pieces are:

• As sanity, I'd be interested in seeing a a test that demonstrates a double map and an NMap(2) which same key types and hashers, generate exactly identical keys, and are thus exactly identical. (Done in Support NMap in generate_storage_alias substrate#9147)

• have a procedural macro for impl_key_prefix_for!((A, B, C), (A, B), C); .... (Done in Convert impl_key_prefix_for to proc macro substrate#8844)

• update generate_storage_alias to support nmap as well (Done in Support NMap in generate_storage_alias substrate#9147)

• maybe making StorageMap and StorageDoubleMap using StorageNMap under the hood.

• maybe consider using storage n map everywhere instead of storage map and storage double map. or the other way around: Implement StorageNMap substrate#8635 (review)

Originally posted by @thiolliere in paritytech/substrate#8635 (comment)