input: Write a python function to find all substrings of a given string.
  assert find_substrings("abc") == ["a", "ab", "abc", "b", "bc", "c"]
  assert find_substrings("xyz") == ["x", "xy", "xyz", "y", "yz", "z"]
  assert find_substrings("a") == ["a"]
output: def find_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