Avoid bad pattern of wildcard fees

[gavofyork]

Alters the two XCM dispatchables so that the BuyExecution uses the first Asset in the list to purchase fees, rather than MultiAsset::All, which is not a pattern that will be supported in the upcoming XCM v1.

Migration

pallet_xcm::Config has an extra item type LocationInverter. This should be the same definition as declared in the xcm_executor::Config, typically:

type LocationInverter = LocationInverter<Ancestry>;

[KiChjang]

Code-wise looks ok, I'm just wondering how we can enforce that the first element in the assets vector is going to be used for fee payment (or whether it is at all desirable in this point in time to have any such kind of enforcement, programmatically or otherwise).