Create USAGE.md to serve as a friendly guide for usage.
Move the libxev and zig-aio event loops to serve as an example of how to
implement a custom event loop in USAGE.md. Remove dependency on these
libraries and all associated examples and source code.
The motivation for this change is to provide a simpler library. The
power to create a custom event loop will always be kept, however the
support burden of maintaining three event loops is not where I want to
focus my energy. The code is small enough that it's easy enough to be
copy/pasted into a project or provided as a third-party library.
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