Add tests and descriptions for the functions in json_util.c

To better distinguish between entry->k and entry->v being const within linkhash, but non-const outside, add lh_entry_v() and lh_entry_k() accessors.
Make lh_entry->k const.

Cause README.md to show up in Doxygen output as the main page.

tests/tests1: fix printf format for size_t arguments

Change %d to %llu and add cast to unsigned long long for size_t arguments,
otherwise compilation will fail with errors like:

test1.c:70:15: error: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Werror=format=]

%zu is avoided to stay compatible with old libc versions (like old Visual
Studio).

Also, fix the types of the variables when building on Windows.
Also, should address issue #214.

Use stdint.h to check for size_t size

Include stddef.h in json_object.h

Add public API to use userdata independently of custom serializer

svn.metaparadigm.com no longer exists so it is not possible to use source from there.

Also, json_object_set_serializer is changed to respect the userdata
and user_delete parameters when to_string_func is NULL.

The user might want to use the userdata for something different, so the
serializer should ignore it by default.

Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.

It is needed for size_t.

If we use json_inttypes.h we are going to fail when builddir != srcdir,
and since JSON_C_HAVE_INTTYPES_H is defined after the configure script
has run, including json_inttypes.h would be the equivalent on including
stdint.h anyway.

This fixes the build of json-c in the GNOME Continuous builder.

Make serialization format of doubles configurable

configure: check realloc with AC_CHECK_FUNCS() to fix cross-compilation.

Use a macro to indicate C99 to the compiler

Fix various potential null ptr deref and int32 overflows

Add utility function for comparing json_objects

Fix issue #221: JSON_C_TO_STRING_NOSLASHESCAPE works incorrectly

Clarify json_object_get_string documentation of NULL handling & return

Tests added.

Do some basic checks on ints, doubles, strings, arrays and "complex" objects.

This fix errors that can happen when ingesting very large JSON files
when hitting the maximum heap size of the process.

The [AC_PROG_CC_C99 macro](https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/C-Compiler.html) sets the complier to the C99 standard if the compiler does not default to such behaviour (albeit gnu99). This is better than specifically hardcoding flags

AC_FUNC_REALLOC is messed up when cross-compiling, ending up in failed
build with "undefined reference to `rpl_realloc'" error.
AC_CHECK_FUNCS will work both with native and cross builds.

fix regression from 2d549662be

Fix possible memory leak and remove superfluous NULL checks before free()