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/vector3i.h

@@ -334,3 +334,6 @@ bool Vector3i::operator>=(const Vector3i &p_v) const {
 void Vector3i::zero() {
 	x = y = z = 0;
 }
+
+template <>
+struct is_zero_constructible<Vector3i> : std::true_type {};