Use proc macros to generate conversion functions for MultiLocation (#3635)

* Use proc macros to generate conversion functions for MultiLocation

* Add compile test and missing conversion cases

* Add common derives for Parent and Ancestor

* Generate conversion functions for MultiLocation v0 via proc macro

* Add type conversion test and fix a bug

* cargo fmt

* Do not hardcode 8 as the number of max parents

* Use map instead of for loops when generating code fragments

* Spelling

* cargo fmt

* More mapping, less for-looping