Make LuaSmartRef properly non-copyable

Broken since 06e2836726, which added LuaSmartRef. That it had absolutely not followed the rule of five at all didn't use to be a problem because it had only ever been in vectors that got resized only once throughout the lifetime of the program >_>

src/lua/LuaSmartRef.h

@@ -1,12 +1,27 @@
 #pragma once
 #include "LuaCompat.h"
+#include <utility>
 
 class LuaSmartRef
 {
 	int ref = LUA_REFNIL;
 
 public:
+	LuaSmartRef() = default;
+	LuaSmartRef(const LuaSmartRef &other) = delete;
 	~LuaSmartRef();
+
+	LuaSmartRef(LuaSmartRef &&other) noexcept : LuaSmartRef()
+	{
+		swap(*this, other);
+	}
+
+	LuaSmartRef &operator =(LuaSmartRef other)
+	{
+		swap(*this, other);
+		return *this;
+	}
+
 	void Assign(lua_State *L, int index); // Copies the value before getting reference, stack unchanged.
 	void Clear();
 	int Push(lua_State *L); // Always pushes exactly one value, possibly nil.
@@ -20,4 +35,10 @@ public:
 	{
 		return ref != LUA_REFNIL;
 	}
+
+	friend void swap(LuaSmartRef &one, LuaSmartRef &other)
+	{
+		using std::swap;
+		swap(one.ref, other.ref);
+	}
 };