variant: add cfl_variant_print function

[nokute78]

This patch is to add print functions and test codes.

• cfl_variant_print(FILE *fp, struct cfl_variant *val)
• cfl_array_print(FILE *fp, struct cfl_array *array)
• cfl_kvlist_print(FILE *fp, struct cfl_kvlist *list)

e.g. cfl_variant_print(stdout, val);