Rededesign iovec handling

[kamalmarhubi]

Since closing #263 on using &[u8] and &mut [u8] directly in readv(2) et al, I've been thinking of alternatives.

My current thinking:

• switch from a type parameter IoVec<T> to a lifetime parameter IoVec<'a>, effectively restricting the type to &'a [u8]
• add an IoVecMut<'a> for the writeable case, which implements Deref<Target=IoVec<'a>>
• implement as many conversion traits as we can

I've got a partial implementation that I started on after #304. I think some tidying up in the socket modules might need to happen first though.