Convert json Reader to iterators

[glinscott]

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 json module encodes invalid unicode code points in test #7611 - error when parsing strings and we hit EOF
• updates definition of whitespace in json should only be the 4 ascii whitespace chars

[huonw]

Looks awesome, but is there a particular reason this uses ~T rather than just T for the rdr field?

[glinscott]

@huonw: no - that is due to my lack of experience in rust. I tried removing the ~ and it compiles fine, and the speed remains the same. From a semantic point of view, will that not cause a copy of T though?

[huonw]

The ~T does too: the T value has to be copied into the memory allocated by the ~ when you create it; moves of the ~T just copy the pointer (so one word), but copies allocate a new ~-box and copy the whole data across.

Most iterator objects are fairly small so the copies (with plain T) aren't expensive and they are unlikely to be a performance bottle neck anyway, unless one is parsing a huge number of very small JSON files. (If the writer of an iterator object is concerned about size, they could wrap it in a ~/@ themselves to make copies cheaper).