std: sys: io: io_slice: Add UEFI types
UEFI networking APIs do support vectored read/write. While the types for UDP4, UDP6, TCP4 and TCP6 are defined separately, they are essentially the same C struct. So we can map IoSlice and IoSliceMut to have the same binary representation.
Since all UEFI networking types for read/write are DSTs, `IoSlice` and `IoSliceMut` will need to be copied to the end of the transmit/receive structures. So having the same binary representation just allows us to do a single memcpy instead of having to loop and set the DST.
cc ``@nicholasbishop``
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