Add a proper implementation of Clone for C API vector types

The previous implementation used a shallow copy, which is incorrect and
could lead to a use-after-free.

crates/c-api/src/vec.rs

@@ -29,7 +29,6 @@ macro_rules! declare_vecs {
         ))*
     ) => {$(
         #[repr(C)]
-        #[derive(Clone)]
         pub struct $name {
             size: usize,
             data: *mut $elem_ty,
@@ -80,6 +79,12 @@ macro_rules! declare_vecs {
             }
         }
 
+        impl Clone for $name {
+            fn clone(&self) -> Self {
+                self.as_slice().to_vec().into()
+            }
+        }
+
         impl From<Vec<$elem_ty>> for $name {
             fn from(vec: Vec<$elem_ty>) -> Self {
                 let mut vec = vec.into_boxed_slice();