- Added the Number.subfactorial() method, which computes the number of derangemenets of a set with n elemements.
- Added the Number.subfactorial() method, which computes the number of derangemenets of a set with n elemements.

See also:
	https://oeis.org/A000166
	https://en.wikipedia.org/wiki/Derangement

Implemented using the formula:

	subfactorial(n) = floor(n!/e + 1/2),   for n>=1

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