Compare fixnums and floats accurately in value<

Compare fixnums and floats accurately in value<

Make `value<` compare fixnums and floats by value, as `<` does, instead
of coercing the fixnum to a float first which is what C would do.
This matters when the fixnum cannot be represented as a float. For
example, C would evaluate

  72057594037927935 < 72057594037927936.0

to false since the operands are converted to the same floating-point
number.

* src/fns.c (fixnum_float_cmp): New.
(value_cmp): Use it.
* test/src/fns-tests.el (fns-value<-ordered, fns-value<-unordered):
New test cases.

(cherry picked from commit f9f4f054bc15791ff63252e101dbf08bb4eb33c1)