Add is_zero_constructible to denote if a type can be semi-trivially constructed with all 0 bytes.

Optimize `CowData` and `LocalVector` resize for zero constructible types.
Mark several compatible types as `is_zero_constructible`.

core/math/plane.h

@@ -132,3 +132,6 @@ bool Plane::operator==(const Plane &p_plane) const {
 bool Plane::operator!=(const Plane &p_plane) const {
 	return normal != p_plane.normal || d != p_plane.d;
 }
+
+template <>
+struct is_zero_constructible<Plane> : std::true_type {};