Found while fuzzing. Previously 1.1897314953572317650857593266280070162E4932
was parsed as +inf, which caused issues for round-trip serialization of
floats. Only f128 had issues, but have added other tests for all
floating point large normals.

The max_exponent for f128 was wrong, it is subtly different in the
decimal code-path as it is based on where the decimal digit should go.
This needs to be 2 greater than the max exponent (e.g. 308 or 4932) to
work correctly (greater by 1, then we use a >= comparision).

In addition, I've removed the redundant `optimize` constant which was only
use for testing the slow path locally.