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/script/lua_api/l_env.cpp

@@ -35,6 +35,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "treegen.h"
 #include "emerge.h"
 #include "pathfinder.h"
+#include "face_position_cache.h"
 
 struct EnumString ModApiEnvMod::es_ClearObjectsMode[] =
 {

src/script/lua_api/l_server.cpp

@@ -26,6 +26,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "environment.h"
 #include "player.h"
 #include "log.h"
+#include <algorithm>
 
 // request_shutdown()
 int ModApiServer::l_request_shutdown(lua_State *L)