Implement TX creation

Updates CLSAG signing as needed. Moves around Error types.

CLSAG multisig and the multisig feature is currently completely borked 
because of this. The created TXs are accepted by Monero nodes.

coins/monero/src/bulletproofs.rs

@@ -0,0 +1,23 @@
+use monero::{consensus::deserialize, util::ringct::Bulletproof};
+
+use crate::{Commitment, transaction::TransactionError, free, c_gen_bp};
+
+pub fn generate(outputs: Vec<Commitment>) -> Result<Bulletproof, TransactionError> {
+  if outputs.len() > 16 {
+    return Err(TransactionError::TooManyOutputs)?;
+  }
+
+  let masks: Vec<[u8; 32]> = outputs.iter().map(|commitment| commitment.mask.to_bytes()).collect();
+  let amounts: Vec<u64> = outputs.iter().map(|commitment| commitment.amount).collect();
+  let res;
+  unsafe {
+    let ptr = c_gen_bp(outputs.len() as u8, amounts.as_ptr(), masks.as_ptr());
+    let len = ((ptr.read() as usize) << 8) + (ptr.add(1).read() as usize);
+    res = deserialize(
+      std::slice::from_raw_parts(ptr.add(2), len)
+    ).expect("Couldn't deserialize Bulletproof from Monero");
+    free(ptr);
+  }
+
+  Ok(res)
+}