Vector2DOps fixes

[thehappycheese]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.
• [x] ran cargo fmt then cargo fmt --all -- --check

• [x] ran cargo clippy --all-features --all-targets -- -Dwarnings

Fixing some derps in my previous PR :)

• Removed Send + Sync from Scalar type; as far as I can tell these constraints are only needed if the trait uses threads. Removing Send+Sync should not prevent a user from using the trait with a Send+Sync type.
• Switched to cmath::hypot for magnitude() for instant gain in robustness, and fixed tests to reflect

[michaelkirk]

Thanks!

bors r+

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• ci result