Make most XCM APIs accept an Into<MultiLocation> where MultiLocation is accepted (#3627)

* Change send_xcm MultiLocation argument to be generic

* Change pallet_xcm::send_xcm MultiLocation and Junctions argument to be generic

* Change convert_origin MultiLocation argument to be generic

* Change OnResponse MultiLocation arguments to be generic

* Change UniversalWeigher MultiLocation argumente to be generic

* Change ExecuteXcm MultiLocation argument to be generic

* Remove usages of into for the MultiLocation argument in execute_xcm

* Make use of generic MultiLocation arguments in rustdocs

* Cargo fmt

* Remove unused import in tests

* Resolve conflicts

* cargo fmt

* Appease spellcheck

* impl Into<MultiLocation> in more places