These all pass now! I have also removed the warning about the LLVM
backend not supporting incremental compilation; I expect it will work
sort of okay in practice by now.
On Windows, it is sometimes problematic to depend on ws2_32.dll. Before,
users of std.Io.Threaded would have to call ioBasic() rather than io()
in order to avoid unnecessary dependencies on ws2_32.dll. Now, the
application can disable networking with std.Options.
This change is necessary due to moving networking functionality to
be based on Io.Operation, which is a tagged union.
Throw another target in there just to spice things up a little!
Running the incremental cases with the C backend is pretty slow due to
the need to recompile the whole output from scratch on every update; for
this reason, we probably don't want to keep many of these targeting CBE
long-term. However, for now, while we have relatively few tests and
things are still changing quite a lot, it's better to have this little
bit of extra test coverage.