v0.3.1

Fixes an out-of-memory crash in estimateDistinct when the source is a Readable
stream. A Readable that hands over one value per read, Readable.from included,
left the loop holding one pending callback per value until the heap ran out. The
promise API now pipes those sources instead of iterating them.

Only estimateDistinct with a Readable was affected. The stream API, the core
CVM, and array or async-iterable sources were not.