- Extended the flt_factor(n) to accept an optional base and the num…

…ber of repetitions to try.

Example (try with base 3 and give up after 10^6 iterations):

	say flt_factor(2**64 + 1, 3, 1e6)	#=> [274177, 67280421310721]

Also optimized the implementation for much better performance.

- Minor code simplifications in the Number class.