[Maxima] Fast conversion to bfloat?

[Raymond Toy]

Raymond Toy wrote:
> After a bit more testing, I'll check in this code.
>   
I've done some testing with random numbers with exponents from 10^5 to
2*10^5.  What I see is that about 4-5% of the time the fast conversion
is off by one bit from the exact conversion.   Can't seem to make it any
better.

The exponent threshold at which the fast conversion is used is
programmable, so perhaps this 5% difference for large enough exponents
is ok.

Ray