Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/support/io_vec.rs

@@ -0,0 +1,45 @@
+use std::io::IoSlice;
+use std::ops::Deref;
+use std::slice;
+
+pub struct IoBufs<'a, 'b>(&'b mut [IoSlice<'a>]);
+
+impl<'a, 'b> IoBufs<'a, 'b> {
+    pub fn new(slices: &'b mut [IoSlice<'a>]) -> Self {
+        IoBufs(slices)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn advance(mut self, n: usize) -> IoBufs<'a, 'b> {
+        let mut to_remove = 0;
+        let mut remaining_len = n;
+        for slice in self.0.iter() {
+            if remaining_len < slice.len() {
+                break;
+            } else {
+                remaining_len -= slice.len();
+                to_remove += 1;
+            }
+        }
+        self.0 = self.0.split_at_mut(to_remove).1;
+        if let Some(slice) = self.0.first_mut() {
+            let tail = &slice[remaining_len..];
+            // Safety: recasts slice to the original lifetime
+            let tail = unsafe { slice::from_raw_parts(tail.as_ptr(), tail.len()) };
+            *slice = IoSlice::new(tail);
+        } else if remaining_len != 0 {
+            panic!("advance past the end of the slice vector");
+        }
+        self
+    }
+}
+
+impl<'a, 'b> Deref for IoBufs<'a, 'b> {
+    type Target = [IoSlice<'a>];
+    fn deref(&self) -> &[IoSlice<'a>] {
+        self.0
+    }
+}