Price feeds for WETH deployment

[kevincheng96]

This PR implements and modifies price feeds to support the upcoming WETH deployment. The favored plan so far is to use ETH-denominated price feeds as opposed to USD price feeds, but stick with using 8 decimals for prices to avoid having to change the Comet and Configurator implementations.

This would require:

• A new wrapper price feed (ScalingPriceFeed.sol) that scales prices up or down to 8 decimals
• A new ConstantPriceFeed that always returns 1e8 for the WETH base asset, since should always hold a 1:1 value with ETH
• Modifications to the WstETHPriceFeed to return prices in terms of ETH instead of USD

This is an alternative approach to #626, which is a more complex change but could be a better long-term solution.

[jflatow]

The description of this PR is out of date now, right? This is now a PR to do truncated USD prices, not ETH

[kevincheng96]

The description of this PR is out of date now, right? This is now a PR to do truncated USD prices, not ETH

The description should still be accurate since this PR is for ETH-denominated prices, not USD. The ScalingPriceFeed can be used for any price denomination, but the changes to WstETHPriceFeed specifically make it denominated in ETH rather than USD.

[kevincheng96]

Updated this PR with a new IPriceFeed interface to be used by the wrapper price feeds and Comet. Discussion can be found here: https://github.com/compound-finance/comet/pull/625#discussion_r1029839340

[jflatow]

Updated this PR with a new IPriceFeed interface to be used by the wrapper price feeds and Comet. Discussion can be found here: #625 (comment)

💪 Thanks for making this change!