Clean up numeric.h and split FacePositionCache from it

I also optiized FacePositionCache a bit: I removed a map
lookup and vector copy from both branches of getFacePosition.

src/CMakeLists.txt

@@ -388,6 +388,7 @@ set(common_SRCS
 	dungeongen.cpp
 	emerge.cpp
 	environment.cpp
+	face_position_cache.cpp
 	filesys.cpp
 	genericobject.cpp
 	gettext.cpp