Parallel should not drop data if paused.

[vqvu]

Yet another parallel bug.

Simplest case:

Given 3 streams s1, s2, and s3 being consumed in this order, when s1 ends, if

• s2 has already ended, and
• the parallel stream is paused (caused by the downstream consumer)

then anything that s3 has already emitted gets dropped on the floor.

For those who care, the problem with the proof sketch in #302 (comment) was the assertion

Otherwise, when the nth stream ends, and it immediately flushes the buffer for the n+1th stream.

This is not true if the nth ended before one of the preceeding streams. Since it wasn't the top stream when it ended, it won't flush the buffer for the n+1th stream.

The fix is obviously make any stream that ends flush as much as it can (not just the next stream's data). Incidentally, this allows us to drop the flush block at the start of the generator, since it's fairly easy to show that there is never anything to flush at that point.