feat: remove IntoIterator requirement on SpendBook trait

It may be too expensive for some trait implementers to support iterating
eg over the network so we decided not to require it, at least for now.

However the default SimpleSpendBook implementation does support iterating.

src/mint.rs

@@ -27,9 +27,7 @@ pub type MintSignatures = BTreeMap<DbcContentHash, (PublicKeySet, NodeSignature)
 
 pub const GENESIS_DBC_INPUT: Hash = Hash([0u8; 32]);
 
-pub trait SpendBook:
-    std::fmt::Debug + Clone + IntoIterator<Item = (DbcContentHash, DbcTransaction)>
-{
+pub trait SpendBook: std::fmt::Debug + Clone {
     type Error: std::error::Error;
 
     fn lookup(&self, dbc_hash: &DbcContentHash) -> Result<Option<&DbcTransaction>, Self::Error>;