XCM: Proper Asset Checking

[gavofyork]

Right now an account is used to represent all assets teleported to other chains (in the case that the assets are minted on the local chain) and to represent all assets teleported in from other chains (in the case that the assets are minted on a remote chain).

In the first case this is a somewhat harmless abuse of the account model, which is useful since a) it's an easy way of managing bookkeeping and b) total issuance is updated naturally to include foreign-held assets.

In the second case it is quite problematic since total issuance double-counts assets held locally as they're teleported in (once in owner's account and a second time in the checking account).

Instead, we should introduce and use a new feature in assets, uniques and balances pallets, exposed by new traits. Other pallets (e.g. ORML) which would want to be fully compatible with the teleportation system can also implement these traits.

The new trait introduces four new functions:

• fn import(checking: AccountId, ... /* asset id, amount &c */): to be called after mint. Note internally that the asset has been imported. Any transfers/burn-and-mints should not alter this mark - the only way to alter the mark should be through the unimport call.
• fn unimport(checking: AccountId, ... /* asset id, amount &c */) -> Result<(), ()>: to be called before burn; return an Err if no outstanding imported asset matches the asset given. Otherwise remove mark as imported.
• fn export(checking: AccountId, ... /* asset id, amount &c */): to be called after burn. Note internally that an asset exists in a foreign system. If total issuance is tracked then it should be increased accordingly. Any transfers/burn-and-mints should not alter this mark.
• fn unexport(checking: AccountId, dest: AccountId, ... /* asset id, amount &c */) -> Result<(), ()>: Remove a mark/note previously placed on an asset by export. If there is no outstanding exported asset, then return an Err.