tokio-util: Remove Encoder bound on FramedParts constructor

[mattfbacon]

Motivation

I cannot construct FramedParts in order to construct Framed if my codec doesn't implement Encoder<I> for some I, despite the fact that I can create a Framed for the same codec, and it will simply not implement Sink.

Solution

Remove the bound.

[Darksonn]

Thanks.

[arcnmx]

Is this not a major breaking change warranting an 0.8? The parameter couldn't be inferred previously, so it was always necessary to explicitly pass it using new::<X>(...) - and now that the parameter was removed, any code using the prior version will no longer compile after a cargo update

[Darksonn]

Yeah, you are right. Removing a where bound is not a breaking change under normal circumstances, and I didn't notice that this particular case had a special generic paramter...