Not supporting partial completions for send, write, read makes
a lot of code paths way simpler, removes Futures from the globalIo and
allows more inheritance.

I don't think partial completion support is worth the hassle.
Partial completions cancel all dependent requests. Resubmitting some
in the chain breaks the semantic.

The global completer does not need to handle requests in its own io_uring.
We don't need templates for submit because submit works only from within
the Runtime. This allows submit to be virtual and be overridden by
Futures which issue a syscall before they submit them selves to the io_uring.

This and the absence of partial completions save a lot of conditions on
the fast path.