Serialize e-cash tokens using our own encoding traits instead of bincode

https://github.com/fedimint/fedimint/blob/13a0e24e902f63fae682c4e4af6e02069621086a/client/client-lib/src/utils.rs#L14-L22

	pub fn parse_coins(s: &str) -> anyhow::Result<TieredMulti<SpendableNote>> {
	let bytes = base64::decode(s)?;
	Ok(bincode::deserialize(&bytes)?)
	}

	pub fn serialize_coins(c: &TieredMulti<SpendableNote>) -> String {
	let bytes = bincode::serialize(&c).unwrap();
	base64::encode(&bytes)
	}

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