Prototype a new Buffer trait.

[sunfishcode]

I'm experimenting with a Buffer trait similar to #908, however I've run into a few problems. See the questions in examples/new_read.rs for details.

@notgull @SUPERCILEX

[SUPERCILEX]

If we go with this approach, I feel like there's a pretty good chance compiler folks will improve the error messages for us if we file bug reports. The story around passing in a mutable array but needing a slice instead has annoyed me for quite some time (I usually just as_mut_slice everything). It was pretty confusing to figure out initially, so I'm sure better error messages would be welcome.

[SUPERCILEX]

Looking at our APIs, I think we'd want to consider adding uninit methods to roughly these methods (the filtering is not great, sorry):

• https://docs.rs/rustix/latest/rustix/?search=%26mut%20%5Bu8%5D
• https://docs.rs/rustix/latest/rustix/?search=Vec%3Cu8%3E

[sunfishcode]

Ok, I've now updated everything in rustix to use the new Buffer trait, except:

• readlink, readlinkat, ptsname, ttyname - These return CString, which is a little different than a buffer of u8.
• The ancillary data API for sendmsg/recvmsg. These are tricky.

I'm not opposed to having them use Buffer, I've just run out of steam for doing them myself. If anyone would like to see them migrated, PRs would be welcome.