In C9X, int16_t *MUST* have exactly 16 bits of precision, but sizeof
can be anything >0.  (Well, any *integer* greater than 0.)

Just a note - not sure if it's how we've done it, but intN_t is *exact*.
For "at least", use "int_leastN_t", I think.

Also note that, as of Friday morning last, <inttypes.h> is now believed
to reserve *all* of int*_t and uint*_t namespaces.  This is intended to
allow implementors some room for expansion.

-s