[API] Allow non-sized writers to be used by serialize_into

[ma2bd]

Slightly generalize the interface serialize_into<W, T> by allowing W: ?Sized.

I came across this issue while trying to write a trait like this one:

trait Writable {
    fn write(&self, writer: &mut dyn std::io::Write) -> Result<()>;
    // ^-- A BCS-based implementation would want to call serialize_into(writer, self)
}

(although there are alternative solutions such as trait Writable<W: std::io::Write>)

[bmwill]

I suspect you'll do a release once this lands?

[ma2bd]

I suspect you'll do a release once this lands?

Yes, that was my intention.