vector: use xy members instead of 01 indices for cross

It's functionally identically, but is easier to read when comparing with
maths documents.

cruft/util/vector.hpp

@@ -66,7 +66,7 @@ namespace cruft {
     T
     cross (vector<2,T> a, vector<2,T> b)
     {
-        return a[0] * b[1] - a[1] * b[0];
+        return a.x * b.y - a.y * b.x;
     }