Added prelude.rs for both v1_6 and v2_0_1 (Used in tests)

[hlsxx]

I’m suggesting this PR because I believe using preludes can be helpful in certain scenarios, including mine 🙂

I’ve also updated tests/schema_validation to reflect how these preludes are used—hope you find it useful!

use crate::v2_0_1::datatypes::prelude::*;
use crate::v2_0_1::enumerations::prelude::*;
use crate::v2_0_1::messages::prelude::*;

[tommymalmqvist]

Nice! Will approve in a couple of days. Need to spend some time on #165 right now

[hlsxx]

Sure, man!