Potential optimization: use an Option

[willemneal]

I wonder if using an Option<i128> could reduce the storage costs down even further.

soroban-core/modules/utxo-core/src/core.rs

Lines 27 to 30 in 3b67db1

	pub enum UtxoState {
	Unspent(i128), // takes 1-byte tag + 16 bytes value
	Spent, // only 1-byte tag (optimizing for read/write size)
	}