Pretty-printing primitive float constants

* map special floats to registered CRef's
* kernel/float64.mli: add {is_infinity, is_neg_infinity} functions
* kernel/float64.ml: Replace string_of_float with a safe pretty-printing function

Namely:

  let to_string_raw f = Printf.sprintf "%.17g" f
  let to_string f = if is_nan f then "nan" else to_string_raw f

Summary:

* printing a binary64 float in 17 decimal places and parsing it again
  will yield the same float, e.g.:

  let f1 = 1. +. (0x1p-53 +. 0x1p-105)
  let f2 = float_of_string (to_string f1)
  f1 = f2

* OCaml's string_of_float gives a sign to nan values which shouldn't be
  displayed as all NaNs are considered equal here.

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