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

coins/monero/src/tests/hash_to_point.rs

@@ -11,6 +11,6 @@ use crate::{
 fn hash_to_point() {
   for _ in 0 .. 50 {
     let point = &random_scalar(&mut OsRng) * &ED25519_BASEPOINT_TABLE;
-    assert_eq!(rust_hash_to_point(point), c_hash_to_point(point));
+    assert_eq!(rust_hash_to_point(point.compress().to_bytes()), c_hash_to_point(point));
   }
 }