Move scriptapi to separate folder (by sapier)

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)

src/script/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_subdirectory(common)
+add_subdirectory(cpp_api)
+add_subdirectory(lua_api)
+
+set(SCRIPT_SRCS 
+	${SCRIPT_COMMON_SRCS}
+	${SCRIPT_CPP_API_SRCS}
+	${SCRIPT_LUA_API_SRCS}
+	PARENT_SCOPE)