Add support for non-interest-bearing assets

[d1ll0n]

Add support for wrapper tokens with a variable conversion rate.

• Store records of the conversion rate with timestamps.
• On a new record being written, take the difference in conversion rate and calculate the average growth per second.
• • Enforce a minimum delay between updates to ensure this isn't manipulated in short time spans.
• Extrapolate growth per token per second to a year.

e.g.
Yearn token has pricePerFullShare of 1e18 today, and 1.005e18 in 10 days. This is a growth of 0.5% in 10 days, making the average interest per second 8.267195767195768e-7 %. Extrapolated to a year, that's 26% APY.