WP 34S and 31S bugs and fixes

[Bit]

I've committed a change to optionally allow entering four-digit exponents in double precision mode. The same set of rules governing exponent entry are available as in single precision mode (except there's no debug option) and rules for double precision mode can be independently configured.

I don't expect this feature to be enabled in mainline as Walter said he didn't like the idea of displaying four-digit exponents on the screen of the 34S. And I'm not sure how much practical value being able to enter four-digit exponents offers as such very large or small numbers probably aren't needed often in practice in a pocket calculator. Nevertheless, since I created the code some time ago, I've added it for the sake of completeness. As usual, it has no cost in terms of firmware size or speed if disabled.

I've attached a 34S flash image and a Windows binary to demo the new features. It has a similar configuration as the previous ones I attached with these exceptions:
- Warnings occupy both lines during number entry so this behavior can be directly compared to the other one.
- Exponents are right justified and padded with zeros so the digits don't move when we switch from three to four digits in the exponent.
- If flag D is set and the mantissa is zero, any exponent can be entered (+/-999 or +/-9999 in double precision mode).