performance: deserialize meta in api::append

[cablehead]

from @marvin-j97 :

xs/src/api.rs

Lines 225 to 236 in a387940

	let meta = match parts
	.headers
	.get("xs-meta")
	.map(|x| x.to_str())
	.transpose()
	.unwrap()
	.map(|s| serde_json::from_str(s).map_err(|_| format!("xs-meta isn't valid JSON: {}", s)))
	.transpose()
	{
	Ok(meta) => meta,
	Err(e) => return response_400(e.to_string()),
	};

you could call serde_json::from_slice directly (if header value derefs to &[u8])