Implement a CountedStorageMap

[shawntabrizi]

This PR introduces a new Storage abstraction: CountedStorageMap

The CountedStorageMap is a simple wrapper around a StorageMap and StorageValue<u32> which keeps count of the number of items in the map by using the value as a counter.

[shawntabrizi]

Few things I need help improving here:

• Is it possible to get rid of the Key and Value generics in CountedStorageMap? The information is redundant in the fact that we wrap StorageMap which ultimately defines the key and value.
• Does it make sense to create a match arm for the #[pallet::storage] macro for this, so we dont need to define the other two storage items, but just define one object?
• How can I prevent users from calling the Map / Value without using the wrapper? It does not seem I can keep things private.

[gui1117]

about raised issue, the whole thing is that we would want to get rid of the generator (which were only useful for decl_storage), in order to implement the traits StorageMap directly.

some idea were given in another context here #8605 (comment)

[gui1117]

if we want to support limit, then we need sp-io to return the number of deleted key in the overlay, once merged I will open an issue, and we can work on it when we need it.

[shawntabrizi]

Is this not already covered here?

#9106

[gui1117]

the new API doesn't say about how many value are removed from the overlay, without this information it is not possible to update the counter efficiently

[gui1117]

superseeded by #9125 for proper review approvals