WP 34S and 31S bugs and fixes

[Bit]

This is not a bugfix but I might as well announce it here: I've refactored set_exp() in display.c to save about 20 bytes in the firmware and created a compile time option (disabled by default) to display all four digits of large exponents in double precision mode. I've added the changes to the 31S even though it has no double precision mode because of the slight reduction in firmware size.