Function `vec_write_all_vectored`

pub(in ::io::cursor) fn vec_write_all_vectored<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, bufs: &[IoSlice<'_>]) -> Result<usize>
where
    A: Allocator,

Resizing write_all_vectored implementation for Cursor.

Cursor is allowed to have a pre-allocated and initialised vector body, but with a position of 0. This means the Write will overwrite the contents of the vec.

This also allows for the vec body to be empty, but with a position of N. This means that Write will pad the vec with 0 initially, before writing anything from that point