Upgrade to WETH10

Right now our rounds are using wxDai, which I believe is WETH9 under the hood.

WETH10 was just released and it includes some nice new features that we could use to improve our user experience. Detailed in this Twitter thread.

Specifically, we could use permit() or transferAndCall() to eliminate the approve() transaction for contributors.

We could also unwrap the assets when recipients claim the assets after a round, meaning that native assets go in and native assets come out and no one has to deal with wrapped tokens.