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ARROW-7924: [Rust] Add sort for float types

This relaxes the trait bound of `std::cmp::Ord` to `std::cmp::PartialOrd` to enable sorting by floats

Closes #7193 from nevi-me/ARROW-7924

Authored-by: Neville Dipale <nevilledips@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>