Pre-allocating buffers and registering them to io_uring

[penberg]

For high performance I/O, it would be great if there was an API to pre-allocate buffers and register them to io_uring as described in https://gavinray97.github.io/blog/io-uring-fixed-bufferpool-zig. Of course, we need to support the same thing for other backends that don't support registering buffers, although pre-allocation anyway helps.