feat: dynamically-sized CpuSet

[SteveLauC]

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What does this PR do

Before this PR, our CpuSet type used only 1 libc cpuset structure, which can only hold 64 bits on both FreeBSD-like and Linux-like systems. With this limitation, it won't work on systems with more than 64 cores, as reported by #2589.

This PR changes the CpuSet type to an enum, which has 2 variants, a sized variant that is same as the previous definition, a dynamic variant that is dynamically allocated, which will extend itself when needed.

Checklist:

• [ ] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API