Refactor

Move json_value and obj_t to same file
 - Rename json_obj.* -> json_value.*
 - Move `print_json_value(...)` to json_value.*

Move `err_ctx(...)` to util.*

parse.h now only exposes `parse_json_value(...)`

include/util.h

@@ -3,6 +3,9 @@
 #define _UTIL_H
 
 #include <stddef.h>
+#include <stdio.h>
+
+__attribute__((__noreturn__)) void err_ctx(int exit_code, FILE* fp, const char* format, ...);
 
 void* malloc_or_die(size_t size);
 void* realloc_or_die(void* ptr, size_t size);