Enable contracts::seal_get_storage to skip reading the actual value

[cmichi]

We recently introduced a thin-wrapped data structure Mapping in ink! and would like to add a .contains(…) function to it's API.
The contracts pallet doesn't have an API to do that at the moment though, so we currently always need to load the full value to check for existence.

@athei suggested to change the seal_get_storage API in a way that the -1 (u32::MAX) sentinel value would skip reading the actual value. He said it's used for other seal_* API's too.

After this issue is implemented we can create a follow-up in ink!.