factor.m: Overhaul function to support inputs > flintmax. * NEWS: Announce improvements in factor. * factor.m: Update documentation to state that for floating point inputs the maximum size is flintmax(). Add input validation to check that input Q is smaller than flintmax for floating point inputs. Change algorithm to operate using the underlying class of the input rather than always converting to double. Remove post-run verification code which is no longer necessary. Update BIST tests.

--- a/NEWS	Tue Aug 17 14:34:42 2021 -0700
+++ b/NEWS	Tue Aug 17 16:28:36 2021 -0700
@@ -180,6 +180,9 @@
 
 - The function `dec2bin` and `dec2hex` now support negative numbers.
 
+- The function `factor` now supports uint64 inputs larger than
+`flintmax`.
+
 - The functions `quantile` and `prctile` now permit operating on
 dimensions greater than `ndims (x)`.