Bug 1938504 - Work around rust < 1.78 crash

Somehow the DWARF info generated by the compiler for the
`hash`-replacement assignment is confusing to LLVM, which crashes.

By using a different form for the same operation, the compiler is happy.

mls-rs/src/tree_kem/parent_hash.rs

@@ -135,7 +135,7 @@ impl TreeKemPublic {
             )
             .await?;
 
-            (parent.parent_hash, hash) = (hash, calculated);
+            (parent.parent_hash = std::mem::replace(&mut hash, calculated);
         }
 
         Ok(hash)