std::io::util cleanup + fixes

[sfackler]

• Fix LimitReader's Buffer::consume impl to avoid limit underflow
• Make MultiWriter fail fast instead of always running through each
  Writer. This may or may not be what we want, but it at least
  doesn't throw any errors encountered in later Writers into oblivion.
• Prevent IterReader's Reader::read impl from returning EOF if given
  an empty buffer.

[breaking-change]

[lilyball]

I'm not convinced the MultiWriter change is appropriate. Does it really matter if the other errors are thrown into oblivion? If they're permanent errors, they'll reoccur on the next write, and if they're temporary errors, then it doesn't seem like a big deal. I guess the question is, who actually uses MultiWriter today, and what behavior is best for them?

[alexcrichton]

For comparison, Go's MultiWriter fails fast.

[lilyball]

Apparently the answer is "nobody uses MultiWriter". At least, nobody in the rust-lang/rust tree. Given that, I withdraw my objection regarding MultiWriter.