Consider EIP-2612 permit() deposit support

[mhluongo]

Newer tokens that support EIP-2612 can make Saddle cheaper and easier to use for users and LPs by removing unnecessary approve() tx's.

USDC, FEI, LUSD, renBTC, tBTC v2, and WETH10 all support EIP-2612 today... meaning swaps and LP'ing involving any of those assets saves 46k gas or more and unnecessary waits for new users, and allows return users to avoid the risk of infinite allowances.

I think this could be a big win.