Make USD* the unit of accounting

[Psyf]

As described by @krenzx, making USD* the unit of accounting makes more sense given the volatility in the space.

As such we need to change this line in the contracts require(_constraints.allowedDepositToken.isETH()); to requiring USDC/USDT (which?).

The bulk of the work is in changing the tests to start at USDC instead of ETH.