Real.maxFinite and friends

[Henry Cejtin]

The only real problem in my mind with making real be 80 bits (and not having
something like Real64 and Real32) is arrays.  I.e., the space occupied.
Also I guess communication with ffi stuff.  I.e., if I only have 80 bit
reals, then I will be upset if I test to see if something isFinite, and
yet when I send it to the foreign code it gets infinity because of the
fact that 64 bit reals have fewer exponent bits.