rt::io: Add Bytes iterator for Reader

[bluss]

An iterator that simply calls .read_bytes() each iteration.

I think choosing to own the Reader value and implementing Decorator to
allow extracting it is the most generically useful. The Reader type
variable can of course be some kind of reference type that implements
Reader.

In the generic form the Bytes iterator is well behaved itself and does not read ahead.
It performs abysmally on top of a FileStream, and much better if a buffering reader is inserted inbetween.

[bluss]

We could add further basic support for Iterator<char> and UTF-8 readers https://github.com/blake2-ppc/rust/compare/reader-iter but it is all too simple (needs to fit into the scheme of buffering, text layer, encoding support?)

[brson]

Added an issue for buffering #8953

[brson]

Thanks! I'm glad you are continuing to push rt::io forward. I haven't had time to work on it much myself lately.

[bluss]

If you think it's ok I think ByteIterator is the better version of that. Also clarified the doc comment on iteration -- conditions make the IO iterators more interesting than the others, but at the moment (PR #8276) there is no stronger guarantees needed for an iterator.

[brson]

@anasazi This overlaps the work you are doing with iteration.