Optimize `check_account_signature` and `account_public_keys`

[abizjak]

Description

Currently these methods in concordium-std accept/return BTreeMaps. THis is not very efficient in the sense that it requires parsing which is not very useful in common cases (since the data is typically just serialized again).

We should have, at least in the low-level host, a way to avoid this serialization. For returning keys that would be in the form of an iterator over keys.