Fix self-move-assignment and leak in ConflictSet move-assignment

The user-declared move-assignment operator overwrote `impl` without first
destroying the existing implementation object, leaking all memory and resources
owned by the left-hand side. Self-move-assignment also set `impl` to nullptr,
leaving the object invalid and leaking the old state.

Fix all three implementations (ConflictSet.cpp, SkipList.cpp, HashTable.cpp)
to guard against self-assignment and to destroy/free the old `impl` before
taking ownership of `other.impl`.

HashTable.cpp

@@ -119,7 +119,13 @@ ConflictSet::ConflictSet(ConflictSet &&other) noexcept
     : impl(std::exchange(other.impl, nullptr)) {}
 
 ConflictSet &ConflictSet::operator=(ConflictSet &&other) noexcept {
-  impl = std::exchange(other.impl, nullptr);
+  if (this != &other) {
+    if (impl) {
+      impl->~Impl();
+      safe_free(impl, sizeof(Impl));
+    }
+    impl = std::exchange(other.impl, nullptr);
+  }
   return *this;
 }