Auto merge of rust-lang#103137 - dtolnay:readdir, r=Mark-Simulacrum

Eliminate 280-byte memset from ReadDir iterator

This guy:

https://github.com/rust-lang/rust/blob/1536ab1b383f21b38f8d49230a2aecc51daffa3d/library/std/src/sys/unix/fs.rs#L589

It turns out `libc::dirent64` is quite big—https://docs.rs/libc/0.2.135/libc/struct.dirent64.html. In rust-lang#103135 this memset accounted for 0.9% of the runtime of iterating a big directory.

Almost none of the big zeroed value is ever used. We memcpy a tiny prefix (19 bytes) into it, and then read just 9 bytes (`d_ino` and `d_type`) back out. We can read exactly those 9 bytes we need directly from the original entry_ptr instead.

## History

This code got added in rust-lang#93459 and tweaked in rust-lang#94272 and rust-lang#94750.

Prior to rust-lang#93459, there was no memset but a full 280 bytes were being copied from the entry_ptr.

<table><tr><td>copy 280 bytes</td></tr></table>

This was not legal because not all of those bytes might be initialized, or even allocated, depending on the length of the directory entry's name, leading to a segfault. That PR fixed the segfault by creating a new zeroed dirent64 and copying just the guaranteed initialized prefix into it.

<table><tr><td>memset 280 bytes</td><td>copy 19 bytes</td></tr></table>

However this was still buggy because it used `addr_of!((*entry_ptr).d_name)`, which is considered UB by Miri in the case that the full extent of entry_ptr is not in bounds of the same allocation. (Arguably this shouldn't be a requirement, but here we are.)

The UB got fixed by rust-lang#94272 by replacing `addr_of` with some pointer manipulation based on `offset_from`, but still fundamentally the same operation.

<table><tr><td>memset 280 bytes</td><td>copy 19 bytes</td></tr></table>

Then rust-lang#94750 noticed that only 9 of those 19 bytes were even being used, so we could pick out only those 9 to put in the ReadDir value.

<table><tr><td>memset 280 bytes</td><td>copy 19 bytes</td><td>copy 9 bytes</td></tr></table>

After my PR we just grab the 9 needed bytes directly from entry_ptr.

<table><tr><td>copy 9 bytes</td></tr></table>

The resulting code is more complex but I believe still worthwhile to land for the following reason. This is an extremely straightforward thing to accomplish in C and clearly libc assumes that; literally just `entry_ptr->d_name`. The extra work in comparison to accomplish it in Rust is not an example of any actual safety being provided by Rust. I believe it's useful to have uncovered that and think about what could be done in the standard library or language to support this obvious operation better.

## References

- https://man7.org/linux/man-pages/man3/readdir.3.html