Tighter space allocation for floating point stringify.
Make pack-as-int/sprintf-%c-ing/chr-ring inf/nan fatal.

In pack: No point in trying to return all-bit-off/all-bits-one
because inf/-inf/nan really don't map sensibly into integers.

In printf-%c/chr: while U+FFFD would be an option, better to die
on such weird input.

pack-as-fp still works, sprintf-numeric still works.

Make t/op/infnan.t to be less fragile about the number of expected tests.