Refactor open interest and oi shares reduction

[magnetto90]

liquidate() and unwind() functions share a considerable amount of code. This PR creates an internal function to avoid these duplicates. Thanks Uncle Bob.

Merge after #133

[TomasCImach]

Consider removing oiTotalOnSide and oiTotalSharesOnSide from the function parameters and fetching these values with:

uint256 oiTotalOnSide = pos.isLong ? oiLong : oiShort;
uint256 oiTotalSharesOnSide = pos.isLong ? oiLongShares : oiShortShares;

or can be made with:

        if (pos.isLong) {
            oiLong = oiLong.subFloor(
                pos.oiCurrent(fraction, oiLong, oiLongShares)
            );
            oiLongShares -= pos.oiSharesCurrent(fraction);
        } else {
            oiShort = oiShort.subFloor(
                pos.oiCurrent(fraction, oiShort, oiShortShares)
            );
            oiShortShares -= pos.oiSharesCurrent(fraction);
        }

The base branch was changed.

[TomasCImach]

pending tests

[magnetto90]

pending tests

Used brownie test for this PR since its a refactor and not a new feature.

The merge-base changed after approval.