partial build system integration of the afl fuzzing tool

This is achieved by encoding the type of function used in the first two
bytes.

First byte:
b: buffered

Second byte:
f: formatted

With one big limitation: It can only be fuzzed with what has been
parsed by the library beforehand.

ensure improvements

We don't need to copy the entire printbuffer, only the part that is
used.

This allowed for the removal of a lot of if (p) checks.

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.