Support more interesting buffer management strategies

[srrrse]

The Read and Write events demonstrated here both use a new, uniquely owned buffer. There are better strategies, which should be supported in an extensible, abstractable way.

One trivial (but maybe not better) solution is to use a buffer pool.

But more interesting solutions are integrated with io-uring itself. It has two different APIs for preregistering buffers that the IO will be performed into. The most interesting is definitely the PROVIDE_BUFFERS operation. This library should support IO operations using those tools to allow the kernel to select buffers itself.