make vecdeque contiguous before shrinking it (#30)

work around a standard library bug

src/mem/queue.rs

@@ -27,9 +27,13 @@ impl RollingBuffer {
     }
 
     fn drain_start(&mut self, pos: usize) {
-        let before_len = self.len();
+        let target_capacity = self.len() * 9 / 8;
         self.buffer.drain(..pos);
-        self.buffer.shrink_to(before_len * 9 / 8);
+        // TODO this is a workarround for https://github.com/rust-lang/rust/issues/108453
+        if self.buffer.capacity() > target_capacity {
+            self.buffer.make_contiguous();
+            self.buffer.shrink_to(target_capacity);
+        }
     }
 
     fn extend(&mut self, slice: &[u8]) {