Rename `unstable-stream` feature to just `stream`

[seanmonstar]

While hyper is pre-1.0, it's not really fair to say other pre-1.0 dependencies are unstable.

Should stream be enabled by default?

[hwchen]

To me, Stream seemed like a core trait, just like Future, so I was surprised when I had to switch it on in hyper.

I know that the reality around what’s core at the moment is a bit more complicated at the moment, but the above is just my initial reaction.

I would vote yes for stream as default, but perhaps there’s downsides I didn’t think of.

[davidbarsky]

I think it should be. Worst comes to worst, Hyper can issue a smaller 0.14 release that walks back on the on-by-default stream feature.

[davidbarsky]

I forgot to note this, but: I think it's reasonable to say that a hypothetical 0.14 release will come out in a half a year to a year's time. I don't believe Stream will be in the standard library six months from now.