Optimize calls to 'eql', 'memql' and similar for fixnums.

Optimize calls to 'eql', 'memql' and similar for fixnums.

It's good practice to compare integers using 'eql' because two bignum
objects representing the same integer might not be 'eq'.  However,
'eql' is slower and doesn't have its own byte code.  Therefore,
replace it with 'eq' if one argument is guaranteed to be a fixnum on
all platforms.

* lisp/emacs-lisp/byte-opt.el (byte-optimize--fixnump): New helper
function.
(byte-optimize-equal, byte-optimize-member, byte-optimize-assoc): Use
it to optimize 'eql' etc. to 'eq' if it will always compare fixnums.