XCM `canonicalize` + `prepend_with` fix (#3269)

* canonicalize + prepend_with fix

* fix doc

* not needed

* better docs, and more deterministic logic

* one more test

* Update xcm/src/v0/multi_location.rs

* Update multi_location.rs

Co-Authored-By: parity-processbot <>

* follow style guide

Co-Authored-By: parity-processbot <>

* oops

* Update xcm/src/v0/multi_location.rs

Co-authored-by: Robert Habermeier <[email protected]>

* Update xcm/src/v0/multi_location.rs

Co-authored-by: Robert Habermeier <[email protected]>