Refactor decoration-related code

Split up ModApiMapgen::l_register_decoration()
Define and make use of CONTAINS() and ARRLEN() macros

src/util/numeric.h

@@ -25,6 +25,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "../irr_v3d.h"
 #include "../irr_aabb3d.h"
 #include <list>
+#include <algorithm>
 
 // Calculate the borders of a "d-radius" cube
 void getFacePositions(std::list<v3s16> &list, u16 d);
@@ -139,6 +140,9 @@ inline v3s16 arealim(v3s16 p, s16 d)
 	return p;
 }
 
+#define ARRLEN(x) (sizeof(x) / sizeof((x)[0]))
+#define CONTAINS(c, v) (std::find((c).begin(), (c).end(), (v)) != (c).end())
+
 // The naive swap performs better than the xor version
 #define SWAP(t, x, y) do { \
 	t temp = x;            \