fix zcachedb logs entry printing (openzfs#826)

When attempting to print chunks/entries (e.g. with `zcachedb logs --index`),
if the `cffi::Slice` can be a direct memory (`Bytes`) reference, then
the binary value of the entries is printed, rather than pretty-printing
them.

This commit provides a better `Debug` implementation for `cffi::Slice`,
which fixes `zcachedb logs`.

cmd/zfs_object_agent/util/src/cffi.rs

@@ -8,6 +8,9 @@
 //! was written out, see block_encode.rs.
 
 use std::any::type_name;
+use std::fmt;
+use std::fmt::Debug;
+use std::fmt::Formatter;
 use std::mem::align_of;
 use std::mem::size_of;
 use std::ops::Deref;
@@ -116,12 +119,23 @@ pub fn bytes_to_slice<T: Cffi>(bytes: Bytes) -> Result<Slice<T>> {
     }
 }
 
-#[derive(Debug)]
 pub enum Slice<T: Cffi> {
     Referenced(Bytes),
     Owned(Vec<T>),
 }
 
+impl<T: Cffi + Debug> Debug for Slice<T> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Slice::Referenced(_) => f
+                .debug_tuple("Slice::Referenced")
+                .field(&self.deref())
+                .finish(),
+            Slice::Owned(_) => f.debug_tuple("Slice::Owned").field(&self.deref()).finish(),
+        }
+    }
+}
+
 impl<T: Cffi> Slice<T> {
     pub fn new_owned(vec: Vec<T>) -> Self {
         Slice::Owned(vec)