Add flatten_left and flatten_right

[CMDJojo]

This resolves #83 by adding flatten_left and flatten_right methods. I did not add a flatten method (like suggested in the issue) due to into_inner doing the same thing. (Also, rustfmt moved imports a bit, that's why those rows were changed)

[cuviper]

First, sorry I never responded here for so long!

Rather than using self.either(identity, Right) and vice versa, you could also write these as:

• flatten_left => self.left_or_else(Right)
• flatten_right => self.right_or_else(Left)

To me, these existing methods already seem sufficient. Are there common cases where nested Either comes up that these really deserve dedicated methods?