- Extended the Number `divisors` method accept an optional argument.
- Extended the Number `divisors` method accept an optional argument.

When an extra argument is given, the method `n.divisors(k)` returns the divisors of `n` that are less than or equal to `k`.

For large or highly composite `n` and relatively small `k` (<= 10^6), the method efficiently finds the divisors of `n`.

Example:

	say 5040.divisors(42)		# divisors of 5040 that are <= 42
	say divisors(2**128 - 1, 1e5)	# divisors of 2^128 - 1 that are <= 10^5

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