eIP 14: Contract Upgrades

Upgrade Euler contracts as per git commit d7f62927eb58592d82302c87205490c26cccca7c.

This set of upgrades fixes some issues and adds new features and optimisations.

These are the highlights:

• Gas-efficient flash loans
• donateToReserves
• Better simulations of batch execution
• Chainlink pricing support

For full details on the changes, as well as code diffs, see the forum post: https://forum.euler.finance/t/eip-14-contract-upgrades/305