Implemented an adaption of CIP2LargestFirst to handle multiassets as
the current implementation would only order by ADA resulting in very
poor selections if many inputs were available and non-ADA multiassets
were in the outputs.

The previous CIP2LargestFirst algorithm will now error on multiasset
outputs.

MultiAsset's documentation was improved slightly and convenience methods
`get_asset()` and `set_asset()` are included now to avoid having to
additionally deal with Assets and do it all at once.