Speed up comparisons between 2 fixnums

Speed up comparisons between 2 fixnums

Since <, <=, > and >= have their own byte-ops, the corresponding
functions are mostly used as arguments to higher-order functions.
This optimisation is particularly beneficial for sorting, where the
comparison function is time-critical.

* src/data.c (Flss, Fgtr, Fleq, Fgeq):
* src/fileio.c (Fcar_less_than_car):
Fast path for calls with 2 fixnum arguments.