std: Flush when buffered writers are dropped

It's still not entirely clear what should happen if there was an error when
flushing, but I'm deferring that decision to #12628. I believe that it's crucial
for the usefulness of buffered writers to be able to flush on drop. It's just
too easy to forget to flush them in small one-off use cases.

cc #12628
std: Switch stdout/stderr to buffered by default

Similarly to #12422 which made stdin buffered by default, this commit makes the
output streams also buffered by default. Now that buffered writers will flush
their contents when they are dropped, I don't believe that there's no reason why
the output shouldn't be buffered by default, which is what you want in 90% of
cases.

As with stdin, there are new stdout_raw() and stderr_raw() functions to get
unbuffered streams to stdout/stderr.
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.