Add init function for BP statics

Considering they take 7 seconds to generate, thanks to #68, the ability 
to generate them at the start instead of on first BP is greatly 
appreciated.

Also performs minor cleanups regarding BPs.

coins/monero/src/ringct/bulletproofs/mod.rs

@@ -41,6 +41,14 @@ impl Bulletproofs {
     len + clawback
   }
 
+  pub fn init(plus: bool) {
+    if !plus {
+      OriginalStruct::init();
+    } else {
+      PlusStruct::init();
+    }
+  }
+
   pub fn prove<R: RngCore + CryptoRng>(
     rng: &mut R,
     outputs: &[Commitment],