Add a method for joining an iterable of `&str`s while appending newlines to those that don't already end with one

[jwodder]

• Technically, the "iterable of &strs" should be I: IntoIterator, I::Item: AsRef<str>.

• Possible parameters (in addition to the iterable):

  • Option 1: A single Newline, to append to those strings that don't already end with it
  • Option 2: A Newline and a NewlineSet; the Newline is appended to those strings that don't already end with a newline in the NewlineSet

• Possible return types:

  • Option 1: Return a String (requires std)
  • Option 2: Return an iterable containing the same elements as the original but with a newline yielded after each string that doesn't already end with a newline (doesn't require std)

Cf. #13