improved parser speed

This improves the parsing speed to some extent. With the test mentioned in #59 what was earlier:

````
file loaded and read:            elapsed time: 1.17439588 seconds
Parsing JSON from string:        elapsed time: 24.428244242 seconds (1512573028 bytes allocated)
Parsing JSON directly from file: elapsed time: 48.749431539 seconds (1691895796 bytes allocated)
Generating JSON:                 elapsed time: 5.412014493 seconds (286913924 bytes allocated)
````

is now:
````
file loaded and read:            elapsed time: 0.440743364 seconds
Parsing JSON from string:        elapsed time: 6.33863731 seconds (685529844 bytes allocated)
Parsing JSON directly from file: elapsed time: 13.676480578 seconds (860866772 bytes allocated)
Generating JSON:                 elapsed time: 3.057185216 seconds (286119136 bytes allocated)
````

It still needs more improvement. Method `parse_string` appears quite prominently in profiler results. Would using substring here be acceptable?