Can't recover signatures when r or s is < 32-bytes

[carver]

This appears to be an eth-lib issue, but it bubbles up here, so should be tested and tracked.

This happens when the r or s component of a signature is 31 bytes or fewer. (assuming uniform distribution, around 1% of the time)

The signature encode/decode in eth-lib assumes that it is always 66 bytes long, and slices up the signature incorrectly, so it cannot recover the sender of the transaction.

I created a test to demonstrate this issue, but can't work on fixing it for a while. I'll attach a PR shortly.