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/Cargo.toml

@@ -28,6 +28,7 @@ curve25519-dalek = { version = "3", features = ["std"] }
 
 group = { version = "0.12" }
 dalek-ff-group = { path = "../../crypto/dalek-ff-group" }
+multiexp = { path = "../../crypto/multiexp" }
 
 transcript = { package = "flexible-transcript", path = "../../crypto/transcript", features = ["recommended"], optional = true }
 frost = { package = "modular-frost", path = "../../crypto/frost", features = ["ed25519"], optional = true }