input: Write a python function to check if one string is a substring of another.
  assert is_substring('foo', 'foobar') == True
  assert is_substring('bar', 'foobar') == True
  assert is_substring('baz', 'foobar') == False
  assert is_substring('foobar', 'foo') == False
output: def is_substring(small, big):
      return small in big