Macro gear_common::wrap_extended_storage_double_map

macro_rules! wrap_extended_storage_double_map {
    (storage: $storage: ident, name: $name: ident, key1: $key1: ty,
        key2: $key2: ty, value: $val: ty, length: $len: ty) => { ... };
}

Same as wrap_storage_double_map!, but with extra implementations of CountedByKey, IterableMap and IterableByKeyMap over double map values.

PrefixIterator from frame_support and KeyValueIteratorWrap from this crate should be in scope.