YIP-82: Move yUSD funds to yDAI Vault

Yearn's yUSD vault was their first product and the only one available when YAM launched. Since then, Yearn has developed numerous other vault products and the yUSD vault no longer earns comparable returns to the newer, more robust and flexible products.

As we rebalance the Treasury and YDS per YIPs 80, and 81, we have an opportunity to upgrade our yield bearing stable-coins to one of these newer products. yDAI is the most philosophically aligned with our mission as DAI is an un-censorable stablecoin backed by ETH and other assets.

Making this change should earn the DAO additional interest from our stablecoins, and although no rates are set in stone, the returns on yDAI should be consistently higher than yUSD due to how the vault is constructed.

See the full post with rationale and data here: https://forum.yam.finance/t/yip-82-transition-yusd-to-new-vault/1493