Faster and more compact bracket matching function

[fangq]

Need a better replacement for the matching_bracket() in loadjson. The current function is very slow in octave (matlab 8.x is ok, despite that profiling still returns high run-time)

https://github.com/fangq/jsonlab/blob/master/loadjson.m#L459-L496

[okomarov]

It would be helpful to have an example to test against.