Updated patch;  applies cleanly to current trunk.  No significant changes.

Note that there's now a new reason to apply this patch: it ensures that 
the result of a long->float conversion is independent of whether we're 
using 30-bit digits or 15-bit digits for longs.

One problem:  if long->float conversions are correctly rounded, then
int->float conversions should be correctly rounded as well.  (And ideally, 
we should have float(int(n)) == float(long(n)) for any integer n.)

This problem only affects 64-bit machines:  on 32-bit machines, all 
integers are exactly representable as floats, and the C99 standard 
specifies that in that case the conversion should be exact.