Add io_uring_buf_ring struct

[ryanseipp]

Closes #1143.

Adds a new type io_uring_buf_ring, defined in the kernel here as:

struct io_uring_buf_ring {
	union {
		/*
		 * To avoid spilling into more pages than we need to, the
		 * ring tail is overlaid with the io_uring_buf->resv field.
		 */
		struct {
			__u64	resv1;
			__u32	resv2;
			__u16	resv3;
			__u16	tail;
		};
		struct io_uring_buf	bufs[0];
	};
};

linux-raw-sys exports the type here, so we can verify alignment and sizes of the types. Additionally, an integration test was added to ensure we can use the structs, anonymous union type, and flexible array member correctly.