* 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 <[email protected]>
Co-authored-by: Giles Cope <[email protected]>
Co-authored-by: parity-processbot <>