Replace all occurences of unsigned int and unsigned long with size_t.
This is a backwards incompatible change, as the signature of many API
functions changes.
Initialize their reference counts to (unsigned int)-1 to disable
reference counting on them. It already was meant to work like this,
but the reference counts were just initialized to 1 instead of -1.
Thanks to Andrew Thompson for reporting this issue.
Added functions are:
* json_string_nocheck()
* json_string_set_nocheck()
* json_object_set_nocheck()
* json_object_set_new_nocheck()
These functions don't check that their string argument is valid UTF-8,
but assume that the user has already performed the check.
