Ensure MultiLocation always has a canonical representation (#3404)

* MultiAsset TWO

* Ensure MultiLocation always has a canonical representation

* Remove v1 module

* Draft next MultiAsset API.

* Implement custom encoding/decoding scheme for MultiLocation

* Properly implement IntoIterator for Junctions

* Implement TryFrom<MultiLocation> for Junctions

* Fix spelling mistakes

* Fix tests in xcm-executor

* XCM core builds

* XCM Executor builds

* XCM Builder builds

* Fix xcm-builder tests and compilation

* Make pallet-xcm compile

* Use MultiLocation::default()

* Make polkadot-runtime-common compile

* Make rococo-runtime compile

* Change return type of parent_count to u8

* Change MAX_MULTILOCATION_LENGTH to 255

* Make kusama-runtime compile

* Fix logic in pallet-xcm

* Use MultiLocation::empty()

* Fix logic in location_conversion

* Fix logic in origin_conversion.rs

* Make westend-runtime compile

* Rename prefixes and suffixes variables

* Rename non_parent to interior

* Rename non_parent to interior

* Add test for encode/decode roundtrip and fix decode algorithm

* API changes making their way throughout

* Some TODOs

* Further build fixes

* Rename non_parent/junctions to interior

* Basic compile builds

* First test fixed

* All executor tests fixed

* Typo

* Optimize subsume_assets and add test

* Optimize checked_sub

* XCM Builder first test fixed

* Fix builder tests

* Fix doc test

* Make xcm-simulator compile

* Make xcm-simulator-example compile

* Make spellcheck happy

* cargo fmt

* fix some doc tests

* spelling

* named fields for AllOf

* Fix subtle bug where Null is treated as an identifier

* Add FIXME comment awaiting for const generics eval stabilization

* Update xcm/src/v0/multiasset.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update xcm/src/v0/multiasset.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update xcm/src/v0/multiasset.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update xcm/src/v0/multiasset.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Reformat

* Move to XCM version 1

* Spelling

* warnings

* Replace some more v0->v1s

* warnings

* format

* Add max_assets param

* building

* test fixes

* tests

* another test

* final test

* Update rustdocs and add debug_assert where sensible

* Revert debug_assert in const fn len()

* tests

* Rename Null -> Here

* Introduce

* More ergonomics

* More ergonomics

* test fix

* test fixes

* docs

* BuyExecution includes

* Fix XCM extrinsics

* fmt

* Make Vec<MultiAsset>/MultiAssets conversions safe

* More MultiAssets conversion safety

* spelling

* fix doc test

* Apply suggestions from code review

Co-authored-by: Amar Singh <asinghchrony@protonmail.com>

* Apply suggestions from code review

Co-authored-by: Amar Singh <asinghchrony@protonmail.com>

* fmt

* Add v0, remove VersionedMultiAsset

* Remove VersionedMultiLocation

* Update xcm/src/v1/order.rs

Co-authored-by: Amar Singh <asinghchrony@protonmail.com>

* Update xcm/src/v1/mod.rs

Co-authored-by: Amar Singh <asinghchrony@protonmail.com>

* XCM v0 backwards compatibility

* Full compatibility

* fmt

* Update xcm/pallet-xcm/src/lib.rs

* Update xcm/src/v0/order.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Tweaks to versioning system

* Fixes

* fmt

* Fix pallet-xcm tests

* fix

* Substitute with_parent with with_parents_const

* Rename argument name from a to m

* Rename split_last to split_last_interior

* Allow adding multiple parents in MultiLocation

* Rename pop_parent to dec_parent

* Ensure relay chain XCM sender receives a MultiLocation without any parents

* Block only when MultiLocation destination length is 8

* Cargo fmt

* Remove reverse iterators, implement DoubleEndedIterator and add tests

* Fix iter_rev lifetime requirements

* Cargo fmt

* Add an into() method for Junctions for conciseness in const context

* Ensure parent count is 0 while executing who in RelayedFrom

* Appease spellchecker

* Use and_then instead of repeated map_err

* Remove custom codec indices for v1 Junctions

* Add convenience 'contains_parents_only' method to MultiLocation

* Fix merge conflict

* Use more convenience methods

* Remove with_parachain_interior

* Prefer matching against tuple instead of using match guards

* Match against tuple instead of using more match guards

* Update encode/decode test for MultiLocation

* Minor tweaks

* Fixes

* Fixes

* Fixes

* Fix MultiLocation

* Add deprecation note for iter_rev and into_iter_rev

* Update some rustdocs

* cargo fmt

* Fix xcm-executor unit tests

* Fix compilation and unit tests in xcm-builder

* cargo fmt

* Fix tests in xcm-simulator-example

* Publicize MultiLocation fields

* Match on the MultiLocation struct directly in xcm-builder

* Do not dereference undereferenceable types

* Add convenience MultiLocation conversions for tuples

* Use clearer import paths

* Remove unused dependency

* fix junction + response

* Import from latest opaque xcm module

* Update xcm/src/v1/mod.rs

* better comment

* Fix ownership transfer

* Fix merge

* Fix merge

* cargo fmt

* Fix merge

* Fix merge

* Fix integration test

* More readable Parent syntax

* cleanup

* cleanup

* cleanup

* cleanup

* cleanup

* cleanup

* cleanup

* cleanup

* cargo fmt

* Fixes

* Fix doc test

Co-authored-by: Gav Wood <gavin@parity.io>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Amar Singh <asinghchrony@protonmail.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>