Return chi from pot.drip()

[epheph]

Related to #87 (potentially incompatible with)

If you already have the pot's chi variable loaded from storage, it is worthwhile to return that value. Many (most?) external callers are likely to need that value immediately after calling pot.drip() (and will need to get that value in a subsequent call). Returning the data already loaded from storage would cost very little gas, a secondary call will cost ~2,000 gas

[epheph]

Nice! 👍