Configure and test whitelisted XCM reserve transfers

[sea212]

This PR adjusts the XCM configuration to only allow the necessary XCM instruction necessary to execute a reserve transfer. Asset teleports are disabled (needs trust).

By default no foreign assets are registered. The AssetRegistry pallet is used to register foreign asset, which are identified by CurrencyId::ForeignAsset(id). The AssetRegistry holds all necessary information about an asset, such as the location, existential deposit, name and the number of decimal fractional places. It also contains some custom metadata, which specifies if the asset can be used in a pool and how fees should be determined. Assets can only be added through governance.
Whenever a foreign asset is received, the XCM implementation checks if it convert it to a local CurrencyId representation. It either maps the Zeitgeist parachain multilocation to CurrencyId::Ztg, or queries the asset registry for that multilocation. In case the conversion succeeds, balance updates will happen accordingly. If the multilocation cannot be resolved, the UnknownTokens pallet will be used to store those transferred tokens. XCM currently uses unidirectional channels and there is no response that lets the sender know if the operation succeeded. When another chain has an open channel to Zeitgeist and sends the XCM to reserve transfer some asset, the sending chain already locally updated the token balances to match the transfer. However, at that point in time it does not know if it will succeed in the destination chain. To avoid Zeitgeist receiving reserve asset transfers and just dropping them because the instructions cannot be handled (token conversion failed), the UnknownTokens pallet keeps track of those unknown tokens.
The fee for XCM transfers is calculated based on a formula that is used across all projects. The fees are always taken from the token that is received, such as KSM. Using the AssetRegistry custom metadata, one can specify a multiplier that is used to adjust fees. For example, if 1 KSM is 10x the worth of 1 ZTG, a 10x multiplier must be specified. Those multiplier can be updated through governance. All the fees are put into the treasury for now.
A Barrier does specify whether XCM instructions should be executed. Such a barrier could include the constraint that execution time must be paid using the BuyExecution XCM instruction. The barriers in place in this PR are restrictive default barriers used by most projects.
Reserve Asset Transfers can be executed by calling the dispatchable transfer from the newly added xTokens pallet. It will ensure the XCM message is crafted properly and send it to the destination chain.

The PR adds integration tests that simulate a relay chain and multiple parachains to verify that only the XCM instructions of interest can be executed and that only the whitelisted chains and assets can be transferred. It tests that transfers to and from another parachain and transfer to and from the parent chains works. It checks that fees are correctly calculated and deducted, that all account balances are updated accordingly, that the sovereign account balances are updated accordingly and that the fees are properly put into the treasury. It also tests that custom fees from the AssetRegistry calculated properly.

[Chralt98]

Just a couple more comments.

[Chralt98]

Just waiting for #850..

[Chralt98]

Ok, let's try that! Happy inter-chain talk.