Auto merge of #93668 - SUPERCILEX:path_alloc, r=joshtriplett

Reduce CString allocations in std as much as possible

Currently, every operation involving paths in `fs` allocates memory to hold the path before sending it through the syscall. This PR instead uses a stack allocation (chosen size is somewhat arbitrary) when the path is short before falling back to heap allocations for long paths.

Benchmarks show that the stack allocation is ~2x faster for short paths:

```
test sys::unix::fd::tests::bench_heap_path_alloc                  ... bench:          34 ns/iter (+/- 2)
test sys::unix::fd::tests::bench_stack_path_alloc                 ... bench:          15 ns/iter (+/- 1)
```

For long paths, I couldn't find any measurable difference.

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I'd be surprised if I was the first to think of this, so I didn't fully flush out the PR. If this change is desirable, I'll make use of `run_with_cstr` across all platforms in every fs method (currently just unix open for testing). I also added an `impl From<FromBytesWithNulError>` which is presumably a no-no (or at least needs to be done in another PR).

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Also see https://github.com/nix-rust/nix/pull/1655 with a bunch of discussion where I'm doing something similar.

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