auto merge of #12630 : alexcrichton/rust/flush-buffered, r=brson

auto merge of #12630 : alexcrichton/rust/flush-buffered, r=brson

Now that we can call `flush()` in destructors, I think that it's appropriate for stdout/stderr to return buffered writers by default.

This doesn't enable certain functionality like a buffered stdin does, but it's what you want 90% of the time for performance reasons.