* Add `swap` and `decode_len` to `DoubleMap`
* Add tests to `swap` and `decode_len` for `DoubleMap` (WIP)
* Address review comments
* Remove function that is not in scope
* fix test
* better naming
* Implement all storage after prefix
* fix test, bump version and fix doc
* bump metadata version
* Update frame/support/procedural/src/storage/storage_struct.rs