Enable the use of [] for StorageMap.

[mohammadfawaz]

It would be nice to be able to do this:

storage {
    map: StorageMap<u64, u64>,
}

...

storage.map[key] += value; // destructures to storage.map.insert(key, storage.map.get(key) + value);

[adlerjohn]

Does this not require enshrining StorageMap into the compiler? Or will this work for anything that implements a storage insertion with a key?

[mohammadfawaz]

Yeah we may need an Index trait like Rust has think 🤔

[otrho]

#429