Added a stream for non-cloneable iterators

[Marwes]

This PR sketches out a possible implementation for a stream which do not directly support cloning. It does so by storing the tokens read from the iterator into a circular buffer which only allows for partial backtracking. Using this additional indirection iterators which cannot be cloned can be used as streams.

The performance drop using this stream type was measured to be about 20% for the JSON parser in the benches folder. Reading directly from a file or socket is not tested but it should work though the chars iterator on Read (though it is currently unstable).

I will hold on to merging this for a while as I would really appreciate some feedback on this as I do not intend to use this myself (and it needs documentation).

cc #30

[hawkw]

This definitely seems like a useful thing to have!

[Marwes]

@hawkw Yeah, it could be implemented outside the library but it is probably useful enough (and isn't trivial to implement) to warrant inclusion.

I realize now that io::Chars has Result<char, CharsError> as items which should proabably be handled but it cannot be done with the current API. It would probably be a good idea to handle the error case as well in some way.

[Marwes]

And one more thing as food for thought for anyone reading this. I used & references to share the buffer for efficiency's sake which is probably what you want in any case, would there be any use for a variant which shares it through Rc instead? (It strikes me as similar to the PR is submitted to zslayton/lifeguard#4 which has both).

A problem with using & references seems to be that the buffered stream must strictly outlive the parsers which consume it (which bit me while writing the tests but shouldn't prevent you from writing any actual code).

[Marwes]

To make sure I can release a stable 1.0 within a week or two I will hold of with this PR until shortly after 1.0 when I will put this and probably range streams in a 1.1-beta version where it can be better tested.

#42