Simplify parsing

This restructures parse_string in a way, that you need to keep less
state in your head to understand the code.

This is achieved by:
* only changing the input pointer (current position) at a few places
(not all throughout)
* splitting out the UTF16 handling into a separate function
* renaming the variables so you know what they do without additional
context

This only attaches the parsed linked lists to the items passed to
parse_object and parse_array.

This replaces the bit fiddling involved with calculating a new buffer
size by just multiplying the required length by two, paving the way to a
complete switch to size_t at a later point in time.

Introduces a preprocessor directive that aborts the compilation if the
version numbers in the header don't match with the numbers in the c
file.

Add new type cJSON_Invalid

This sets the type of an item only if parsing was successful.

This means that in case of failure, the item's type will remain to be
cJSON_Invalid.

This assigns the macro cJSON_Invalid to 0.

This fixes a double free that happens when calling cJSON_Delete on an
item that has been used by parse_string and it failed parsing the
string.

The double free happens, because parse_string frees an alias of
item->valuestring, but doesn't set item->valuestring to NULL.

Thanks @rrvirtual for the suggestion.

Introduce the cunity testing framework

Enabling this flag enables AddressSanitizer and
UndefinedBehaviorSanitizer