backport __pairs metamethod to our tweaked version of Lua (#1615)

* check for user-defined __pairs in luaB_pairs, defaults to luaB_next

* go back to passing in luaB_next as an upvalue as "light C functions" are not supported

* add explainer documentation comment

rts/lib/lua/src/lbaselib.cpp

@@ -235,11 +235,23 @@ static int luaB_next (lua_State *L) {
 }
 
 
+/*** 
+ * Meta <code>__pairs</code> backported from Lua 5.2,
+ * originally <code>lua_pushvalue(L, lua_upvalueindex(1)); </code> was <code>lua_pushcfunction(L, luaB_next);</code>
+ * <br>
+ * but that requires "light C function" support,
+ * which makes lua_pushcfunction not allocate extra memory for the GC to clean up.
+ */
 static int luaB_pairs (lua_State *L) {
-  luaL_checktype(L, 1, LUA_TTABLE);
-  lua_pushvalue(L, lua_upvalueindex(1));  /* return generator, */
-  lua_pushvalue(L, 1);  /* state, */
-  lua_pushnil(L);  /* and initial value */
+  luaL_checkany(L, 1);
+  if (luaL_getmetafield(L, 1, "__pairs") == LUA_TNIL) {  /* no metamethod? */
+    lua_pushvalue(L, lua_upvalueindex(1));  /* return generator, */
+    lua_pushvalue(L, 1);  /* state, */
+    lua_pushnil(L);  /* and initial value */
+  } else {
+    lua_pushvalue(L, 1);  /* argument 'self' to metamethod */
+    lua_call(L, 1, 3);  /* get 3 values from metamethod */
+  }
   return 3;
 }