Implement Debug for many types

[brson]

We generally implement Debug for all types in libraries that are intended for general use.

Add Debug for HostAndPort, ParseOptions, PathSegmentsMut, SocketAddrs, UrlQuery, ByteSerialize, Parse, ParseIntoOwned, Serializer, *_ENCODE_SET, PercentEncode, PercentDecode.

[SimonSapin]

Guideline question: some of these types only exist (or are only public) because the language does not support -> impl Iterator<Item=Foo> yet. When it does, will it be recommended that everything uses -> impl Iterator<Item=Foo> + Debug?

[dtolnay]

Which types are you considering replacing with impl Trait? In any case, I filed rust-lang/api-guidelines#60 to follow up on a guideline.

[brson]

Since these do exist today, it is reasonable that they implement Debug, right?

[clarfonthey]

Moving to impl Iterator would be a breaking change anyway, so it doesn't hurt to implement Debug for 1.0.

[TheFirstLairron]

I would be willing to help out with implementing these. Is there anything I would need to know before hopping in?