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`.
weaselbotandandrew