Fix double-rounding bug in ceiling etc.
This is doable now that we have bignums. * src/floatfns.c (integer_value): Remove; no longer used. (rescale_for_division): New function. (rounding_driver): Use it to divide properly (by using bignums) even when arguments are float, fixing a double-rounding FIXME. * src/lisp.h (LOG2_FLT_RADIX): Move here ... * src/timefns.c (frac_to_double): ... from here. * test/src/floatfns-tests.el (big-round): Add a test to catch the double-rounding bug.