Add an epoll::wait_uninit.

[sunfishcode]

This is like epoll::wait, except that it can read into an uninitialized buffer, and it returns a pair of slices, similar to read_uninit. This also means it doesn't require "alloc".

This is a possible alternative direction to #1290. I'm on the fence still. I like some things about the Buffer trait magic, including how it can encapsulate the unsafety of the set_len to resize a Vec to the number of initialized bytes. However, I don't like how it seems to feel more magical than it is. In read(fd, extend(&mut v)), you still have to think about end-of-stream, EINTR, short reads, and spare capacity.