explicit string lengths API modifs

Added:

json_t *json_string_ex(const char *value, size_t len, size_t steal);
json_t *json_string_nocheck_ex(const char *value, size_t len, size_t steal);
size_t json_string_len(const json_t *string);
int json_string_set_ex(json_t *string, const char *value, size_t len, size_t steal);
int json_string_set_nocheck_ex(json_t *string, const char *value, size_t len, size_t steal);

Very minor modifs of changes originally offered by chipdude.

Signed-off-by: Olivier Langlois <olivier@olivierlanglois.net>