refactor: reduce noise in fmt::Debug outputs

[seanmonstar]

I was doing some debugging again, and again reminded that some of these things just print way too much worthless noise. Such as:

h2::proto::streams::streams drop_stream_ref; stream=Stream {
	id: StreamId(187),
	state: State { inner: Closed(Error(Reset(StreamId(187), REFUSED_STREAM, Remote))) },
	is_counted: false,
	ref_count: 1,
	next_pending_send: None,
	is_pending_send: false,
	send_flow: FlowControl { window_size: Window(65535), available: Window(0) },
	requested_send_capacity: 0,
	buffered_send_data: 0,
	send_task: None,
	pending_send: Deque { indices: None },
	next_pending_send_capacity: Some(Key { index: SlabIndex(94), stream_id: StreamId(189) }),
	is_pending_send_capacity: true,
	send_capacity_inc: false,
	next_open: None,
	is_pending_open: false,
	is_pending_push: false,
	next_pending_accept: None,
	is_pending_accept: false,
	recv_flow: FlowControl { window_size: Window(65535), available: Window(65535) },
	in_flight_recv_data: 0,
	next_window_update: None,
	is_pending_window_update: false,
	reset_at: None,
	next_reset_expire: None,
	pending_recv: Deque { indices: None },
	is_recv: true,
	recv_task: None,
	push_task: None,
	pending_push_promises: Queue { indices: None, _p: PhantomData<h2::proto::streams::stream::NextAccept> },
	content_length: Omitted
}

(I indented that to help myself in this PR)

The most interesting changes here are that on the Stream type, many fields are only printed if they are in an interesting state. Such as if the boolean is true, or the option is Some. This does mean that if you care about the field always being there, this will be different. But I think it's a significant improvement.

This is the revised output:

h2::proto::streams::streams drop_stream_ref; stream=Stream {
	id: StreamId(187),
	state: Closed(Error(Reset(StreamId(187), REFUSED_STREAM, Remote))),
	is_counted: false,
	ref_count: 1,
	send_flow: FlowControl { window_size: Window(65535), available: Window(0) },
	requested_send_capacity: 0,
	buffered_send_data: 0,
	next_pending_send_capacity: Some(Key { index: SlabIndex(94), stream_id: StreamId(189) }),
	is_pending_send_capacity: true,
	recv_flow: FlowControl { window_size: Window(65535), available: Window(65535) },
	in_flight_recv_data: 0,
	is_recv: true,
	content_length: Omitted
}