Add 'Stream.ToPull.unconsMin', remove 'Stream.ToPull.unconsLimit' ass…

[seigert]

I've added Stream.ToPull.unconsMin and changed logic of both Stream.ToPull.unconsN and Stream.chunkMin to use it.

Main motivation: if you working with some 'frames' of size N, sometimes you also can process chunks of M * N size in a batch, right now it is possible only via (afaik) .chunkMin(N).scan(Chunk.empty)(..).unchunk. New operation allows this directly on pull.

Also I've removed an assertion of n > 0 on unconsLimit as all other similar methods (unconsN, chunkMin, chunkM) to do not fail and return empty output instead.

---

On a side note, usage of both repeatPull and unconsFlatMap in chunk* operators and other methods is confusing at best. Especially as 'unconsFlatMap' to not use the operations defined in ToPull.

For example, current implementation of unconsLimit will fail on negative n while chunkLimit will produce infinite stream of empty chunks.

[mpilquist]

Looks great!