Statemint Reserve Asset Transfer (#552)

* add AssetId type alias in statemint-common

* add FungiblesAdapter to allow for asset transactions via XCM

* use custom Polkadot

* fix FungiblesAdapter usage and add CheckingAccount

* update deps

* remove polkadot overrides

* update deps

* pull NonZeroIssuance struct into common + add FungiblesTransactor to Statemine and Westmint

* remove unnecessary tuple wrapping + adjust asset transactor comments

* accept statemint as reserve in rococo test parachain

* adjust parachain config (add Statemint as reserve)

* add test and docs for AssetsFrom

* cargo fmt

Co-authored-by: Ricardo Rius <ricardo@parity.io>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>