Fix Issue #1273

Added Lua index registry clear routine to `destroy` method of classes that use it.
If this is left to GC, the abandoned userdata would stay in memory for one full extra cycle.

Prettified the code slightly. Clarified some comments. Refactored `Fixture` class `udata` name.
Removed Reference class include from `Shape` (it doesn't use it).

src/common/Reference.cpp

@@ -45,7 +45,7 @@ Reference::~Reference()
 
 void Reference::ref(lua_State *L)
 {
-	unref(); // Just to be safe.
+	unref(); // Previously created reference needs to be cleared
 	pinnedL = luax_getpinnedthread(L);
 	luax_insist(L, LUA_REGISTRYINDEX, REFERENCE_TABLE_NAME);
 	lua_insert(L, -2); // Move reference table behind value.