fix: delete all non-test uses of `usingnamespace`

[robbielyman]

this PR refactors libxev to avoid using usingnamespace in favor of manually importing the relevant decls into their corresponding structs. i believe this change involves zero changes to the public API of libxev. in particular, the same functions should be exported, and their arguments have not changed. zig build test on my mac completes successfully, but of course i can only run so many of the tests locally.

i have retained the instances of usingnamespace involving test decls, partly because it is not immediately clear to me how to do without it, and partly because they don't, for instance, make using ZLS more difficult.