Improve trait bounds over iterators

[maurolacy]

Currently there's a mess of trait bounds over the different containers. Raw iterators don't need KeyDeserialize, by example, and they usually have that restriction.

Do a full analysis of the required trait bounds for raw and typed iterators, for all the iterable containers:

• Map.
• SnapshotMap.
• IndexedMap.
• IndexedSnapshotMap.

Extend that to the indexes:

• UniqueIndex.
• MultiIndex.

Analyse also the trait bounds over all the public methods of these containers, so that they don't list extra / unneeded bounds. key() by example, requires PrimaryKey but not KeyDeserializable.

Also, the Bounder trait bound is optional. It can be imported through an use clause when needed, but don't need to be listed as a bound over the containers' methods (see CosmWasm/cw-plus#692).