Bug fixes and small API improvements

[zoejbare]

Bug fixes:

• Fixed a memory leak in StringData
• Fixed StringData not handling null C-string inputs
• Fixed GetEmpty() returning a local when the index operator that calls it returns a reference.

API improvements:

• Added a few functions that seemed to be missing
• Renamed the internal string_view class to StringView so it can be supported alongside std::string_view
• Implemented a suite of constructors and assignment operators so users can use the API as described in the readme (i.e., foo["bar"]["baz"][3] = 1.5, which wouldn't compile before)
• Changed the behavior of the non-const string & size_t index operators so that when operating on a JSONObject of type Empty, it would be automatically converted to the Object or Array type to give users an easier option for constructing json data
• Changed the non-const size_t index operator to automatically resize an array if the supplied index is outside the array's current range