Make buffered.Reader.append accept any ByteSeq.

[jemc]

This is an issue of principle-of-least-surprise, since buffered.Writer
emits ByteSeqs in its output, so having buffered.Reader accept ByteSeqs
at its input makes it symmetrical and least surprising.

This doesn't break any user code, since it only makes an argument type
more general, and does not make any return types more general.

This came up in a discussion in #1639.

[aturley]

@jemc looks like this doesn't quite work. Let me know if you want to talk about the options.

I like the idea of applying the principle-of-least-surprise, but I also think that the example in #1639 was pretty contrived so it shouldn't be the main driver of what we're doing here.

[jemc]

Odd, I didn't see those failures locally - something must have been screwy with my local environment, since those are legitimate failures.

[jemc]

Amended to take a slightly different approach in dfb9914.

[jemc]

@Praetonus mentioned to me that we might also want to consider making buffered.Writer return an Array[Array[U8]] instead of an Array[ByteSeq]. Unlike this PR, that would be a breaking change, and would potentially incur a bit of small overhead for the Writer.write and Writer.writev methods if they still accepted a ByteSeq and had to convert to an Array[U8] internally. Also, due to that change being more invasive it would probably be best to do it through RFC.

Still, I think it's at least worth considering that idea before merging this one.