PositionValue may suffer from underflow in 0.8 version

[jes16jupyter]

Hi, according to #198, the solc version of PositionValue is restricted below 0.8.0, as fee calculation code relies on unchecked math that is expected to overflow. Current implementation is pragma solidity >=0.6.8 <0.8.0;.

But in 0.8 branch, PositionValue has no unchecked nor the code is altered, but the version 0.8.0 is included.

Thus unexpected revert will happen as I am understanding it correctly.