MultiLocationsFilter to additionally filter by asset

[sander2]

#714 introduced a filter for xcm locations to the xtokens pallet. However, it checks only the multilocation in isolation, which means that each destination is either supported for all currencies, or that it's not supported at all. However, some destinations only make sense for certain currencies, e.g. sending the chain's native token to the relay chain probably does not make a lot of sense. So I thought it would make sense to change MultiLocationsFilter to take a tuple of a destination and an asset. Thoughts?

[xlc]

Sounds reasonable. cc @Dengjianping