"(nan-payload nan) -- Returns the remaining 51 bits of nan as an
positive exact integer."

Is this for NaN tagging, and can you use those bits without knowing the
float byte order? I presume if you store and retrieve the payload in the
same OS process the byte order stays constant so you don't need to care
about it. But the SRFI doesn't have a procedure to store a payload.

The sample implementation uses `(bytevector-ieee-double-set! bvec 0 n
(endianness big))` from R6RS.

Does toggling the byte order on bi-endian architectures affect the float
byte order as well, or does the FPU have its own byte order?