token transfers

[reyalabs]

CR: I don't suggest we make this change now, but for V2 a significanlty more efficeint way of doing this would be to transfer the funds straight into the marginEngine, and have the marginEngine check that they exist (e.g. that its balance is larger than it can account for). This is how Uniswap works I think, and would allow even more logic to move out of the core contracts without compromising safety. But too late to change for V1.

Originally posted by @CyclopsRex in #101 (comment)