Upgrade from uring-sys to uring-sys2

[neithernut]

At this time, uring-sys did not receive any updates in three and a half years. Open PRs seemingly never got merged, including one PR supplying a fix enabling uring-sys being built on musl-systems. For now, we have to assume it's abandoned.

Luckily, a drop-in replacement which still receives updates and fixes exists in the form of uring-sys2, a fork of uring-sys. This PR switches over to uring-sys2. In addition, it adds SetupFlags and SetupFeatures constants which were introduced with newer versions of liburing.