I prefer factorial as a method (like Ruby and Smalltalk).  Given the
usual notation (n!) or pronounciation (n factorial), it is natural to
write this as:  n.factorial().

Compared to  numbits() and isqrt(), a factorial() method is more basic
in that it is self explanatory and everyone knows what it means by the
time they are in middle school.

FWIW, if separate RFEs were opened for numbits() and isqrt(), I would
support their being int methods too.  Their implementations are helped
by access to the underlying representation, and a case could be made
that these no argument calls are just properties of the number (just
like the sign bit).