Fixed free_sstring to properly free SString.
Added a new struct to make returning and handling multiple SStrings easier.
Mostly removed the split_sstring and changed the definition a bit.
Added a find_str_in_sstring() to replace strstr().
Added a test for find_str_in_sstring().
Forgot to declare SStrings with struct before them.
Fixed the test's arguments.
Forgot to pass string from sub_str instead of itself in find_str_in_sstring.
Assigned return value of strstr so that it hopefully doesn't get optimized out.