Implement StorageNMap (#8635)

* Implement StorageNMap

* Change copyright date to 2021

* Rewrite keys to use impl_for_tuples instead of recursion

* Implement prefix iteration on StorageNMap

* Implement EncodeLike for key arguments

* Rename KeyGenerator::Arg to KeyGenerator::KArg

* Support StorageNMap in decl_storage and #[pallet::storage] macros

* Use StorageNMap in assets pallet

* Support migrate_keys in StorageNMap

* Reduce line characters on select files

* Refactor crate imports in decl_storage macros

* Some more line char reductions and doc comment update

* Update UI test expectations

* Revert whitespace changes to untouched files

* Generate Key struct instead of a 1-tuple when only 1 pair of key and hasher is provided

* Revert formatting changes to unrelated files

* Introduce KeyGeneratorInner

* Add tests for StorageNMap in FRAMEv2 pallet macro

* Small fixes to unit tests for StorageNMap

* Bump runtime metadata version

* Remove unused import

* Update tests to use runtime metadata v13

* Introduce and use EncodeLikeTuple as a trait bound for KArg

* Add some rustdocs

* Revert usage of StorageNMap in assets pallet

* Make use of ext::PunctuatedTrailing

* Add rustdoc for final_hash

* Fix StorageNMap proc macro expansions for single key cases

* Create associated const in KeyGenerator for hasher metadata

* Refactor code according to comments from Basti

* Add module docs for generator/nmap.rs

* Re-export storage::Key as NMapKey in pallet prelude

* Seal the EncodeLikeTuple trait

* Extract sealing code out of key.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>