Convert json Reader to iterators

This is much faster for strings, and eventually when there is a
buffered reader of some sort.

Reading example.json 100 times before was around 1.18s.
After:
- reading from string 0.68s
- reading from file 1.08s (extra time is all in io::Reader)
auto merge of #7704 : glinscott/rust/json_iter, r=erickt

This is much faster for strings, and eventually when there is a
buffered reader of some sort, will be much faster for files.

Reading example.json 100 times before was around 1.18s.
After:
- reading from string 0.68s
- reading from file 1.08s (extra time is all in io::Reader)

Also:
- fixes #7611 - error when parsing strings and we hit EOF
- updates definition of whitespace in json should only be the 4 ascii whitespace chars