Remove unnecessary allocations in message serialization

[joshua-spacetime]

I.e. address the following TODO left by @Centril.

SpacetimeDB/crates/core/src/client/messages.rs

Lines 29 to 65 in c4e637e

	/// Serialize `msg` into a [`DataMessage`] containing a [`ws::ServerMessage`].
	///
	/// If `protocol` is [`Protocol::Binary`],
	/// the message will be conditionally compressed by this method according to `compression`.
	pub fn serialize(
	msg: impl ToProtocol<Encoded = SwitchedServerMessage>,
	protocol: Protocol,
	compression: Compression,
	) -> DataMessage {
	// TODO(centril, perf): here we are allocating buffers only to throw them away eventually.
	// Consider pooling these allocations so that we reuse them.
	match msg.to_protocol(protocol) {
	FormatSwitch::Json(msg) => serde_json::to_string(&SerializeWrapper::new(msg)).unwrap().into(),
	FormatSwitch::Bsatn(msg) => {
	// First write the tag so that we avoid shifting the entire message at the end.
	let mut msg_bytes = vec![SERVER_MSG_COMPRESSION_TAG_NONE];
	bsatn::to_writer(&mut msg_bytes, &msg).unwrap();
	// Conditionally compress the message.
	let srv_msg = &msg_bytes[1..];
	let msg_bytes = match ws::decide_compression(srv_msg.len(), compression) {
	Compression::None => msg_bytes,
	Compression::Brotli => {
	let mut out = vec![SERVER_MSG_COMPRESSION_TAG_BROTLI];
	ws::brotli_compress(srv_msg, &mut out);
	out
	}
	Compression::Gzip => {
	let mut out = vec![SERVER_MSG_COMPRESSION_TAG_GZIP];
	ws::gzip_compress(srv_msg, &mut out);
	out
	}
	};
	msg_bytes.into()
	}
	}
	}