Bulletproofs+ (#70)

* Initial stab at Bulletproofs+

Does move around the existing Bulletproofs code, does still work as 
expected.

* Make the Clsag RCTPrunable type work with BP and BP+

* Initial set of BP+ bug fixes

* Further bug fixes

* Remove RING_LEN as a constant

* Monero v16 TX support

Doesn't implement view tags, nor going back to v14, nor the updated BP 
clawback logic.

* Support v14 and v16 at the same time

coins/monero/src/rpc.rs

@@ -10,6 +10,7 @@ use serde_json::json;
 use reqwest;
 
 use crate::{
+  Protocol,
   transaction::{Input, Timelock, Transaction},
   block::Block,
   wallet::Fee,
@@ -98,6 +99,37 @@ impl Rpc {
     })
   }
 
+  pub async fn get_protocol(&self) -> Result<Protocol, RpcError> {
+    #[derive(Deserialize, Debug)]
+    struct ProtocolResponse {
+      major_version: usize,
+    }
+
+    #[derive(Deserialize, Debug)]
+    struct LastHeaderResponse {
+      block_header: ProtocolResponse,
+    }
+
+    Ok(
+      match self
+        .rpc_call::<_, JsonRpcResponse<LastHeaderResponse>>(
+          "json_rpc",
+          Some(json!({
+            "method": "get_last_block_header"
+          })),
+        )
+        .await?
+        .result
+        .block_header
+        .major_version
+      {
+        13 | 14 => Protocol::v14,
+        15 | 16 => Protocol::v16,
+        _ => Protocol::Unsupported,
+      },
+    )
+  }
+
   pub async fn get_height(&self) -> Result<usize, RpcError> {
     #[derive(Deserialize, Debug)]
     struct HeightResponse {