Add methods for reading into unitialized buffers

[notgull]

An alternative to #908 that uses separate functions to enable reading into uninitialized bytes.

So far I've added uninitialized methods for io methods, net methods, and rand. There are other functions that take &mut [u8], but those aren't as pressing for now.

cc #81

[sunfishcode]

The overall shape here looks good! If, in the future, the standard library adopts some convention for uninitialized buffers, we can follow it.

What would you think about making the doc comments for these new foo_uninit functions just say something like "Like foo, but read into a potentially-uninitialized buffer instead" or so, instead of copying all the documentation content? I know I've done a fair amount of doc-comment copy and pasting myself, but I've been appreciating how shorter comments like this serve to highlight the difference, rather than oblige the reader to carefully compare to full doc comments to find the differences.

[notgull]

What would you think about making the doc comments for these new foo_uninit functions just say something like "Like foo, but read into a potentially-uninitialized buffer instead" or so, instead of copying all the documentation content?

I did this. I left behind the header explaining which syscall it corresponds to.

[notgull]

@sunfishcode When do you think that you'll be able to review this? I want to implement rustix support for some other crates, but MaybeUninit support is a must for these use cases.

[sunfishcode]

Sounds good! I'll merge this, do some additional testing, and then cut a new release.

[sunfishcode]

This is now released in rustix 0.38.27.