Remove pdqselect dependency

[Stoeoef]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

Pros:

• Fewer dependencies!
• Switching to select_unstable (rather than stable) should be faster. I can run a few benchmarks to back this up with numbers if desired. A stable sort is not required for the algorithm to work correctly

Cons:

• Bumps the minimal supported rust version (MSRV) to 1.49

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What's the thought about the MSRV? Is it okay to rely on 1.49? V1.49 was released on 2020-12-31

[Stoeoef]

Thanks! bors r+

[bors[bot]]

Build failed:

• ci result

[Stoeoef]

bors r+

[bors[bot]]

Build succeeded:

• ci result