XCM docs and tests (#2948)

* WIP

* add tests and docs for DoubleEncoded

* reformat parent_count

* add test for match_and_split

* fix append_with docs and add tests

* move Parachain enum variant to tuple

* Fix stuff

* add to append test

* simplify match

* formatting

* format and extend doc comments (including examples)

* fix typo

* add some doc comments

* add test for location inverter

* Add more tests/docs

* Fix build

* matches fungibles

* currency adapter.

* add more tests for location inverter

* extract max length magic number into constant

* adapters.

* Apply suggestions from code review

* Final touches.

* Repot and fixes

* Remove last todo

* Apply suggestions from code review

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

* Update xcm/xcm-builder/src/barriers.rs

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

* Update xcm/xcm-builder/src/barriers.rs

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

* Update xcm/xcm-builder/src/currency_adapter.rs

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

* Update xcm/xcm-builder/src/filter_asset_location.rs

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

* Update xcm/xcm-builder/src/matches_fungible.rs

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

* Update xcm/xcm-executor/src/traits/conversion.rs

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

* Update xcm/xcm-executor/src/traits/conversion.rs

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

* Update xcm/xcm-executor/src/traits/transact_asset.rs

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

* Update xcm/xcm-executor/src/traits/should_execute.rs

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

Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>