Exercise 5.5. Write functions which:
- Compare two strings for equality. If they are equal, zero is returned, otherwise the difference in value between the first two non-matching characters.
- Find the first occurrence of a specific character in a given string. Return a pointer to the occurrence in the string, or zero if it is not found.
- Take two strings as arguments. If the first exists in the second as a substring, return a pointer to the first occurrence, otherwise zero.