Use std::io::IoSlice[Mut] on AsyncRead/AsyncWrite take2 #1533
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This replaces #1476.
iovecfeature already in the FCP, so I think it's good to wait until it's stable.Stabilized in rust-lang/rust#60334.
Also, this renames
poll_vectored_*topoll_*_vectoredto make it in the same order as std.Closes #1455