WP 34S and 31S bugs and fixes

[Bit]

Displayed numbers weren't rounded correctly in double precision mode in SCI/ENG 10/11 display modes with SHOW_LARGE_EXPONENT enabled. For example, pi*10²⁰⁰⁰ shows the problem. The default builds are unaffected because they don't display large exponents.

I've committed a fix, which has a minor beneficial effect on the default builds, too: Y register display is faster (in all display modes) because fewer iterations of set_x_dn() are needed on average to get the number of displayed digits right.