Implement Bulletproofs in Rust (#69)

* Initial attempt at Bulletproofs

I don't know why this doesn't work. The generators and hash_cache lines
up without issue. AFAICT, the inner product proof is valid as well, as
are all included formulas.

* Add yinvpow asserts

* Clean code

* Correct bad imports

* Fix the definition of TWO_N

Bulletproofs work now :D

* Tidy up a bit

* fmt + clippy

* Compile a variety of XMR dependencies with optimizations, even under dev

The Rust bulletproof implementation is 8% slower than C right now, under 
release. This is acceptable, even if suboptimal. Under debug, they take 
a quarter of a second to two seconds though, depending on the amount of 
outputs, which justifies this move.

* Remove unnecessary deref in BPs

crypto/dalek-ff-group/src/lib.rs

@@ -264,6 +264,12 @@ impl PrimeFieldBits for Scalar {
   }
 }
 
+impl Sum<Scalar> for Scalar {
+  fn sum<I: Iterator<Item = Scalar>>(iter: I) -> Scalar {
+    Self(DScalar::sum(iter))
+  }
+}
+
 macro_rules! dalek_group {
   (
     $Point: ident,