Add support for writing to embedded_io::Write

[elrafoon]

Hello,

I think this feature is quite useful for many users.

It's feature-gated, so shall be harmless for others.

[elrafoon]

Dear maintainers, is there any problem with this one?

[eldruin]

Thank you for this PR.
I am not familiar enough with this crate to judge these changes, though.
Maybe @jordens has an opinion?

[jordens]

• I'm worried by the dyn SerializerBackend. serde_json::Serializer is just generic over io::Write. That would be better.
• It's missing the analogous Deserializer changes.

[ryan-summers]

I'm not sure I fully understand the use case. Can you provide a small example of what you're looking to do in code that this enables?

[elrafoon]

In meantime I got better idea. We could add our own Read and Write traits, and then implement those traits for embedded_io::{Read, Write}. Serializer would be generic over Read and Deserializer over Write.

It would look like here