XCM v1 (#2815)

* MultiAsset TWO

* Draft next MultiAsset API.

* XCM core builds

* XCM Executor builds

* XCM Builder builds

* API changes making their way throughout

* Some TODOs

* Further build fixes

* 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

* fix some doc tests

* spelling

* named fields for AllOf

* 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

* 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

* Update xcm/xcm-executor/src/assets.rs

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

* Update xcm/xcm-executor/src/assets.rs

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

* Grumbles

* Update xcm/src/v1/multiasset.rs

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

* fmt

* Update xcm/src/v1/multiasset.rs

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

* Update xcm/src/v1/multiasset.rs

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

* Fixes

* Formatting

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>