WP 34S and 31S bugs and fixes

[Bit]

I've noticed some inconsistencies in how exponents can be entered in single precision mode and on the 31S:

1. Changing the sign
   
   You cannot enter e-384 in the following way because the calculator limits the negative exponent to -383:
   [EEX] [+/-] [3] [8] [4]
   
   However, you can enter it like this as exponents are not checked when the sign is changed:
   [EEX] [3] [8] [4] [+/-]
   
   This is inconsistent behavior, entering e-384 should either be allowed or not.
   
   
2. Domain errors and infinities
   
   Entering 12e384 will produce a domain error or an infinity depending on flag D because the number overflows. I consider this inconsistent behavior, too, because if infinities can be entered this way, I don't see why 1e399 shouldn't be allowed.
   

While experimenting with this, I created two patches:
A. Keep it simple: exponents up to +/- 384 are allowed. No digits are lost even with e-384 because the 12 digits you can enter always fit into the 16 digit mantissa as a denormalized number.
B. Figure out the precise maximum based on the number entered and only allow exponents that will neither result in infinities or domain errors, nor in entered digits lost.

Walter, since this is a user interface issue with no obvious "best" solution, please make a decision on how it should be handled. I don't mind creating a slightly different third patch if necessary.