Buffered I/O wrappers

The default buffer size is the same as the one in Java's BufferedWriter.

We may want BufferedWriter to have a Drop impl that flushes, but that
isn't possible right now due to #4252/#4430. This would be a bit
awkward due to the possibility of the inner flush failing. For what it's
worth, Java's BufferedReader doesn't have a flushing finalizer, but that
may just be because Java's finalizer support is awful.

The current implementation of BufferedStream is weird in my opinion, but
it's what the discussion in #8953 settled on.

Closes #8953