Support StorageMap and StorageVec in complex *storage* types

[mohammadfawaz]

For example, it would be nice to be able to write:

struct Wrapper {
    map1: StorageMap<u64, u64> = StorageMap {},
    map2: StorageMap<u64, u64> = StorageMap {},
}

storage {
    wrapper: Wrapper = Wrapper {
        map1: StorageMap {},
        map2: StorageMap {},        
    },
    tuple_wrapper: (StorageVec<u64>, StorageVec<bool>) = (StorageVec {}, StorageVec {}),
    // ... enum example
    // ... array example (once arrays are supported in storage)
}

This requires the __get_storage_key() intrinsic to pass in more information such as the field index in a struct and so on.

This is a follow up on #1796 which is being closed with #2453. #2453 disallow the above until we implement it.