Square root trait and implementation cleanups

Some potential cleanups for the traits and implementation:

• Add a CheckedSquareRoot trait which could be implemented by Int (returning none for negative inputs). This could be a supertrait of SquareRoot.
• Rename the SquareRoot trait for clarity, perhaps UintSquareRoot or FloorSquareRoot.
• [ ] Implement the square root trait(s) for Montgomery forms (this makes FloorSquareRoot a less desirable name).
• Rename Uint::sqrt/sqrt_vartime to isqrt or sqrt_unchecked to reflect the inexact nature of the result (and corresponding trait updates to match).
• Implement square root traits and methods for OddUint for consistency. This would require changing the return type, but it seems reasonable to add an Output type to the trait(s).
• Support nth roots based on algorithm 1.14 from the same source (Brent & Zimmermann, Modern Computer Arithmetic, v0.5.9). Should this be a separate trait?

Start by reviewing the existing square-root traits and the Uint::sqrt/sqrt_vartime and OddUint APIs, then compare the proposed trait names and return types with the issue's remaining tasks. Done would require an agreed design and implementation for OddUint square roots, plus a decision on whether nth roots belong in a separate trait and how algorithm 1.14 applies.