fix no-alloc build

capnp/src/private/arena.rs

@@ -348,8 +348,8 @@ where
 
     fn size_in_words(&self) -> usize {
         let mut result = 0;
-        for seg in &self.inner.segments {
-            result += seg.allocated as usize
+        for ii in 0 .. self.inner.segments.len() {
+            result += self.inner.segments[ii].allocated as usize
         }
         result
     }