dcutr: fix clippy lints

[elenaf9]

Description

Fix clippy warnings in libp2p-dcutr: new-without-default, single-match, large-enum-variant, redundant-pattern-matching and redundant-closure.

[thomaseizinger]

ACK modulo one thing.

[thomaseizinger]

Personally, I always allow this one because the Box is quite the ergonomic hit when pattern matching and the lint desc. also says that one would need to measure the performance benefit gained from boxing.

[elenaf9]

I agree that in some cases it makes more sense to allow this lint rather than using a Box.
That being said, in this case here we pass around the inbound_connect quite a bit without using it, and for all enums it appeared on this error was thrown. We can wrap it once in a Box and pass that around instead, without any ergonomic hit. The only place where inbound_connect is used is in

rust-libp2p/protocols/dcutr/src/handler/relayed.rs

Lines 371 to 373 in f85a990

	if let Some(Poll::Ready(result)) = self.inbound_connect.as_mut().map(|f| f.poll_unpin(cx)) {
	self.inbound_connect = None;

where we have to use AsMut anyway to access the inner future.

[mxinden]

Thanks Elena!