extflonums

An extflonum is like a flonum, but with 80-bit precision and
not a number in the sense of `number?': only operations such as
`extfl+' work on extflonums, and only on platforms where extflonums
can be implemented by hardware without interefering with flonums
(i.e., on platforms where SSE instructions are used for
double-precision floats).

[Patch provided by Michael Filonenko and revised by Matthew.]

The compiler tracks information about bindings that are known to
hold extflonums, but the JIT does not yet exploit this information
to unbox them (except as intermediate results).