BLI: support multiple arguments for value in *_as methods of Map

This allows us to build more complex values in-place in the map.
Before those values had to be build separately and then moved into the map.

Existing calls to the Map API remain unchanged.

source/blender/blenlib/tests/BLI_map_test.cc

@@ -604,6 +604,15 @@ TEST(map, GenericAlgorithms)
   EXPECT_EQ(std::count(map.keys().begin(), map.keys().end(), 7), 1);
 }
 
+TEST(map, AddAsVariadic)
+{
+  Map<int, StringRef> map;
+  map.add_as(3, "hello", 2);
+  map.add_as(2, "test", 1);
+  EXPECT_EQ(map.lookup(3), "he");
+  EXPECT_EQ(map.lookup(2), "t");
+}
+
 /**
  * Set this to 1 to activate the benchmark. It is disabled by default, because it prints a lot.
  */