NCC audit fixes

[fjarri]

Some small things that were picked up by the security audit.

• Removed conditionals in Uint::saturating_add and saturating_sub and replaced them with constant-time methods.
• Made checks in the random_mod test more logical
• Marked Uint::sqrt to be renamed to sqrt_vartime (and similarly checked_sqrt and wrapping_sqrt) on the next breaking release, since that's the crate's convention.