Consider flash lending collaterals held in the `Join` contracts

[alcueca]

I drafted a module that can flash lend collateral held in the Join contracts. It could be merged with flash.sol to provide the additional functionality.

If the borrower specifies the WETH9 or WETH10 token addresses, it would be nice if the contract would have a path that takes Ether from the EthJoin (if it is used) and wraps it into WETH9 or WETH10 to flash lend.

A vatGemFlashLoan function might also be interesting, where the module just creates the gem in vat for the borrower, does the callback, and then removes the gem from vat.