XcmContext
buy_weight / refund_weight
* added XcmContext to WeightTrader trait * cargo fmt * make xcm context optional * make compile * fix compile * `XcmContext` to `buy_weight / refund_weight` --------- Co-authored-by: Patricio Napoli <hi@patricionapoli.dev> Co-authored-by: Giles Cope <gilescope@gmail.com> Co-authored-by: parity-processbot <>