Replace Vec<Bulletproofs> with Bulletproofs

Monero uses aggregated range proofs, so there's only ever one Bulletproof. This
is enforced with a consensus rule as well, making this safe.

As for why Monero uses a vec, it's probably due to the lack of variadic typing
used. Its effectively an Option for them, yet we don't need an Option since we
do have variadic typing (enums).

coins/monero/src/wallet/send/mod.rs

@@ -658,11 +658,7 @@ impl SignableTransaction {
             pseudo_outs: vec![],
             commitments: commitments.iter().map(|commitment| commitment.calculate()).collect(),
           },
-          prunable: RctPrunable::Clsag {
-            bulletproofs: vec![bp],
-            clsags: vec![],
-            pseudo_outs: vec![],
-          },
+          prunable: RctPrunable::Clsag { bulletproofs: bp, clsags: vec![], pseudo_outs: vec![] },
         },
       },
       sum,