Simplify Debug display of DebugName

[chescock]

Objective

As noted in #21856, the Debug output of DebugName is too verbose. It is supposed to be a thin wrapper around a string, but it renders as a struct.

Solution

Manually impl Debug for DebugName and write the string directly.

Showcase

The following code

#[derive(Debug)]
struct TestStruct {
    debug_name: DebugName,
}
let test_struct = TestStruct {
    debug_name: DebugName::type_name::<TestStruct>(),
};
println!("{test_struct:#?}");

Prints the following before this change

TestStruct {
    debug_name: DebugName {
        name: "crate_name::TestStruct",
    },
}

And the following after it

TestStruct {
    debug_name: "crate_name::TestStruct",
}

When the debug feature is disabled, it prints the following both before and after the change

TestStruct {
    debug_name: DebugName,
}

[janhohenheim]

Maybe tell the user they can enable the debug feature to read the actual name?

[chescock]

Maybe tell the user they can enable the debug feature to read the actual name?

Yeah, that's a good idea! I had been trying to keep it consistent with the old version, but you're right that it's better to include the message. That also simplifies the implementation, since it can just use Deref.

[ickshonpe]

Looks good to me, maybe with the debug feature prompt suggested by janhohenheim.