Auto merge of rust-lang#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.

---

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).

---

Also see nix-rust/nix#1655 with a bunch of discussion where I'm doing something similar.

Author: bors

tests/fail/shims/fs/isolated_file.stderr

@@ -10,6 +10,9 @@ LL |         let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode a
    = note: inside closure at RUSTLIB/std/src/sys/PLATFORM/fs.rs:LL:CC
    = note: inside `std::sys::PLATFORM::cvt_r::<i32, [closure@std::sys::PLATFORM::fs::File::open_c::{closure#0}]>` at RUSTLIB/std/src/sys/PLATFORM/mod.rs:LL:CC
    = note: inside `std::sys::PLATFORM::fs::File::open_c` at RUSTLIB/std/src/sys/PLATFORM/fs.rs:LL:CC
+   = note: inside closure at RUSTLIB/std/src/sys/PLATFORM/fs.rs:LL:CC
+   = note: inside `std::sys::PLATFORM::small_c_string::run_with_cstr::<std::sys::PLATFORM::fs::File, [closure@std::sys::PLATFORM::fs::File::open::{closure#0}]>` at RUSTLIB/std/src/sys/PLATFORM/small_c_string.rs:LL:CC
+   = note: inside `std::sys::PLATFORM::small_c_string::run_path_with_cstr::<std::sys::PLATFORM::fs::File, [closure@std::sys::PLATFORM::fs::File::open::{closure#0}]>` at RUSTLIB/std/src/sys/PLATFORM/small_c_string.rs:LL:CC
    = note: inside `std::sys::PLATFORM::fs::File::open` at RUSTLIB/std/src/sys/PLATFORM/fs.rs:LL:CC
    = note: inside `std::fs::OpenOptions::_open` at RUSTLIB/std/src/fs.rs:LL:CC
    = note: inside `std::fs::OpenOptions::open::<&std::path::Path>` at RUSTLIB/std/src/fs.rs:LL:CC