new string transformations: s.fill.left / s.fill.right

Fills a string with a char/string until the given final length is reached.
The initial string is returned if its length is greater or equal to the
given final length.

Examples:

$var(in) = "485"; (also works for integer PVs)

	* $(var(in){s.fill.left, 0, 6})    => 000485
	* $(var(in){s.fill.left, abc, 8})  => bcabc485
	* $(var(in){s.fill.right, 0, 6})   => 485000
	* $(var(in){s.fill.right, abc, 8}) => 485abcab