Add function to serialize to a Writer

[Boscop]

Bincode has serialize_into which supports an arbitrary Writer, this would be very useful for RON as well, especially with BufWriter, because the data gets very large.

I'm seeing huge RAM spikes because of RON serialization: From ~90 MB baseline RAM usage to over 700 MB when it's serializing. Btw, the resulting ron file is 20 MB large.

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In the above application, I'm serializing app state to disk every 10s.
Btw, the CPU usage is also quite high. This is the CPU & RAM usage when serializing to bincode instead. With ROM, the CPU usage is almost always at ~27% (probably because it takes so long), with bincode, it only shows short spikes (to 24%) when serializing:

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Anyway, with a function like serialize_into+BufWriter we could at least reduce the RAM spikes.
This would also allow chaining it with e.g. a Gzip encoder for streaming compression, e.g. like with bincode:

	bincode::serialize_into(
		GzBuilder::new()
			.write(BufWriter::new(File::create(path)?), Compression::best()),
		&data,
	)?;