Remove bool casting, and make arg optional

[duianto]

The multi-line function has a TODO note:
https://github.com/IvanMalison/multi-line/blob/d5ae863ced0adeb7032ada398005f27a6c669d79/multi-line.el#L227-L234

A possible solution

This seems to make the bool casting obsolete:

(defun multi-line (&optional single-line)
  "Multi-line the statement at point.

When SINGLE-LINE is provided, single-line the statement at point instead."
  (interactive "P")
  (multi-line-execute multi-line-current-strategy single-line))

It also makes the single-line argument optional,
which makes it possible to call (multi-line) without any arguments,
from for example the Eval: (S-M-;) prompt.