Add Cargo feature query_encoding_2 to use encoding_rs crate

[talklittle]

Fixes #444

---

This change is 

[hsivonen]

See my old PR on the same topic for a more copy-avoiding implementation of decode.

[talklittle]

Thanks @hsivonen. I just copied your old Cow code, hope you don't mind.

[hsivonen]

I don't mind. I pointed to it so that you'd copy it. Thanks.

[talklittle]

Closing temporarily. I want to fix cargo build --all-features. I'll probably make the new crate override the legacy one if both feature flags are provided. [Edit: Nevermind, that doesn't make sense since the problem is when 2 libraries are building with conflicting feature flags. I'll have to make both sets of code usable.]

I don't think I'll go the trait approach, since IMO that seems like overkill when AFAIK there are no plans to make a third encoding crate. And if there were another encoding implementation, there's no guarantee it would fit nicely into the existing encoding API.

cc @SimonSapin