AP_Common: add template to check structure size

This has the advantage of telling you what the sizes are

In file included from ../../libraries/AC_AttitudeControl/AC_PosControl.h:3:
../../libraries/AP_Common/AP_Common.h:103:3: fatal error: static_assert failed "wrong size"
  static_assert(s == t, "wrong size");
  ^             ~~~~~~
../../libraries/AP_Common/AP_Common.h:147:52: note: in instantiation of template class 'assert_structure_size<12, 11>' requested here
assert_structure_size<sizeof(struct Location), 11> _assert_location_size;

libraries/AP_Common/AP_Common.h

@@ -100,6 +100,20 @@
 // @}
 
 
+// assert_storage_size template: assert that the memory used to store an
+// item is of a specific size.
+// example invocation:
+// assert_storage_size<class Location, 16> _assert_storage_size_Location;
+// templates are used for this because the compiler's output will
+// usually contain details of the template instantiation so you can
+// see how the actual size differs from the expected size.
+template<typename s, int s_size, int t> struct _assert_storage_size {
+    static_assert(s_size == t, "wrong size");
+};
+template<typename s, int t> struct assert_storage_size {
+    _assert_storage_size<s, sizeof(s), t> _member;
+};
+
 ////////////////////////////////////////////////////////////////////////////////
 /// @name	Types
 ///