input: Write a python function to find all substrings of a given string.
  assert find_all_substrings("abc") == ["a", "ab", "abc", "b", "bc", "c"]
  assert find_all_substrings("abcd") == ["a", "ab", "abc", "abcd", "b", "bc", "bcd", "c", "cd", "d"]
  assert find_all_substrings("a") == ["a"]
output: def find_all_substrings(s):
      substrings = []
      for i in range(len(s)):
          for j in range(i + 1, len(s) + 1):
              substrings.append(s[i:j])
      return substrings