Node placement / mineral / serialization / iron freq / node_dig callback

- Node placement code moved to Lua
- Mineral system removed (added default:stone_with_coal and default:stone_with_iron).
- MapBlock and MapNode serialization updated.
- Mapgen: Frequency of iron increased.
- node_dig callback and related changes.

src/CMakeLists.txt

@@ -115,7 +115,6 @@ set(common_SRCS
 	nodemetadata.cpp
 	serverobject.cpp
 	noise.cpp
-	mineral.cpp
 	porting.cpp
 	materials.cpp
 	defaultsettings.cpp