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/util/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(UTIL_SRCS
+	${CMAKE_CURRENT_SOURCE_DIR}/directiontables.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/numeric.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/pointedthing.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/serialize.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/string.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/timetaker.cpp
+	PARENT_SCOPE)