Implement auto-parallelization for derivationStrict

[edolstra]

Motivation

This makes derivationStrict traverse its dependency graph in parallel in the background.

Example speedups:

• nix eval ~/Dev/nix#devShells.x86_64-linux.native-clangStdenv: 4.06s -> 2.01s.
• nix eval nixpkgs#firefox: 2.67s -> 1.88s .

Context

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch auto-parallelize

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