Opt-in native getters and setters for "newtypes"/"value types"

[dzfranklin]

My goal is to incrementally make the generated code easier to use without getting bogged down in something ambitious (#136). This is intended for simple types that nearly primitive values.

To opt-in you add an annotation and implement a trait

struct User {
    id @0 :Uuid;
}

struct Uuid $Rust.valueType {
  d1 @0 :UInt32;
  d2 @1 :UInt16;
  d3 @2 :UInt16;
  d4 @3 :UInt64;
}

use uuid::Uuid;

impl capnp::traits::ValueType for account_capnp::uuid::Owned {
    type Type = Uuid;

    fn read(reader: Self::Reader<'_>) -> capnp::Result<Self::Type> {
        let d4 = reader.get_d4().to_be_bytes();
        Ok(Uuid::from_fields(reader.get_d1(), reader.get_d2(), reader.get_d3(), &d4))
    }

    fn build(mut builder: Self::Builder<'_>, value: Self::Type) -> capnp::Result<()> {
        let (d1, d2, d3, d4) = uuid.as_fields();
        let d4 = u64::from_be_bytes(*d4);
        builder.set_d1(d1);
        builder.set_d2(d2);
        builder.set_d3(d3);
        builder.set_d4(d4);
        Ok(())
    }
}

let message = capnp::message::Builder::new_default();
let root = message.init_root::<account_capnp::user::Builder>();

let admin_id = Uuid::from_str("a1b2c3d4-e5f6-7a8b-9c0d-1e2f3a4b5c6d").unwrap();
root.set_id(admin_id);
assert_eq!(root.reborrow_as_reader().get_id(), admin_id);

Fixes #245

[dzfranklin]

I've ended up thinking this isn't worthwhile