Allow trait objects in the to_writer function.
Allow trait objects in the to_writer function.

Write is an object safe trait, so users can construct an &mut Write
object. However, the to_writer and to_writer_pretty functions take an
&mut W: Write. In order to allow users to pass &mut Write objects to
these functions, their W parameter is now constrained ?Sized, to allow
it to be a dynamically sized type (such as a trait object).

serde-rs/json,bolinfest/serde-jsonrc,google/serde_json_lenient