- fix bounds checks for exponent range of denormalized numbers
- use correct rounding precision for denormalized numbers
- added extra tests

Change-Id: I6be56399afd0d9a603300a2e44b5539e08d6f592
Reviewed-on: https://go-review.googlesource.com/8096Reviewed-by: Alan Donovan <adonovan@google.com>