Experiment with conversions between integers and floats.

Experiment with conversions between integers and floats.

Switch some conversions from libem calls to inline code.  The
conversions from integers to floats are now too slow, because each
conversion allocates 4 or 5 registers, and the register allocator is
too slow.  I might use these slow conversions to experiment with the
register allocator.

I add the missing conversions between 4-byte single floats and
integers, simply by going through 8-byte double floats.  (These
replace the calls to nonexistant functions in libem.)

I remove the placeholder for fef 4, because it doesn't exist in libem,
and our language runtimes only use fef 8.