ConstChoice improvements

[fjarri]

• A leftover from Normalize the usage of prefixes for method names #417 - rename ct_choice module to const_choice, to match the struct name
• Fixes Use wide arithmetic in CtChoice #418 (in a way) - now Limb's subtle impls piggyback on ConstChoice, so that we don't have two different algorithms doing the same thing. I use the Hacker's Delight approach - it is marginally safer, and doesn't seem to lead to performance reduction.
• Fixes Make a const equivalent of subtle::CtOption #422 - add ConstOption and use it across the API.

I thought about adding NonZero::const_new_unchecked() and ConstOption::unwrap_unchecked() for use in tight loops, but using expect doesn't seem to decrease performance.