tokenise vault

[antoncoding]

The tradeoff here is: is it really a useful feature to transfer the vault? or do we want to focus on gas optimisation

Using sub-account structure similar to Euler let us skip an additional SLOAD that check owner of a "tokenId"

[antoncoding]

Since now we want to focus on the FullMargin and spread implementation, tokenising the position would be slightly more meaning full because people might want to trade their short positions.
But with the new architecture, this would also implies that each MarginEngine can decide individually if they want to tokenise their own position.