Double rounding fix is (still) insufficient #2150

Found during the development of WAH with a binary script converted from const.wast. Minimal reproduction:

(module (func (export "f") (result f32) (f32.const +8.8817847263968443574e-16)))
(assert_return (invoke "f") (f32.const +0x1.000002p-50))

(Note that a similar construction in hexadecimal literals, (f32.const +0x1.00000100000000001p-50), correctly rounds to 0x1.000002p-50.)

The value in question is almost in the middle of two consecutive binary32 fp number 0x1.0p-50 and 0x1.000002p-50, but it is not the exact middle and in fact very slightly closer to 0x1.000002p-50:

d=8.88178419700125232338905334472656250000000000e-16
 +0.00000052939559203401094665527343750000000000e-16
----------------------------------------------------
n=8.88178472639684435740000000000000000000000000e-16
 +0.000000529395592033864477180129688349552452..e-16
----------------------------------------------------
u=8.881785255792436391264477180129688349552452..e-16

The root cause is the inaccuracy of fxx.ml line 285. String.length s here is 25 (20 significant digits + . + e-16), so the format string is %.25g, but %g is platform-dependent! (source) In my experience, msvcrt's printf notably does not give any more significant digits than the type's own limit (even %.100f gives ~18 significant digits followed by zeroes), so %.25g actually doesn't make any difference with msvcrt. The correct approach would be to use an actual bigint (scaled by, say, 5^1100 to elide fractional digits).