Add IoUring::submit_all()

[asayers]

Sometimes you have a bunch of SQEs you want to submit. They may or may not fit in the squeue, but either way you just want to keep submitting in a loop until all the SQEs are submitted.

This PR adds a submit_all() method which does this for you. It's named in analogy with std::io::Write::write_all(), which does something similar.

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It's possibly more correct to check for and ignore ErrorKind::Interrupted when calling submit(). I haven't seen this error happen in practice so I'm not sure.