libiou: add libiou submodule

libiou is a thin veneer over io_uring defining an ergonomic async
IO-oriented API.

Since jio shouldn't need to do anything computationally
intensive, the combination of thunk_h for closures and
libiou+io_uring for scheduling closures according to IO
completions should be sufficient for an implementation.  Though
it may prove annoying to not have per-task stacks and the ability
to arbitrarily yield and/or delay execution pending completion of
non-IO results, we'll see.

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