Update librustzcash crate to use ff crate

Cargo.lock

@@ -227,6 +227,7 @@ dependencies = [
  "bellman 0.1.0",
  "blake2-rfc 0.2.18 (git+https://github.com/gtank/blake2-rfc?rev=7a5b5fc99ae483a0043db7547fb79a6fa44b88a9)",
  "byteorder 1.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "ff 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.40 (registry+https://github.com/rust-lang/crates.io-index)",
  "pairing 0.14.2",

librustzcash/Cargo.toml

@@ -15,6 +15,7 @@ crate-type = ["staticlib"]
 
 [dependencies]
 bellman = { path = "../bellman" }
+ff = "0.4"
 libc = "0.2"
 pairing = { path = "../pairing" }
 lazy_static = "1"

librustzcash/src/rustzcash.rs

@@ -1,6 +1,7 @@
 extern crate bellman;
 extern crate blake2_rfc;
 extern crate byteorder;
+extern crate ff;
 extern crate libc;
 extern crate pairing;
 extern crate rand;
@@ -13,10 +14,8 @@ mod hashreader;
 #[macro_use]
 extern crate lazy_static;
 
-use pairing::{
-    bls12_381::{Bls12, Fr, FrRepr},
-    BitIterator, PrimeField, PrimeFieldRepr,
-};
+use ff::{BitIterator, PrimeField, PrimeFieldRepr};
+use pairing::bls12_381::{Bls12, Fr, FrRepr};
 
 use sapling_crypto::{
     circuit::multipack,

librustzcash/src/tests/key_agreement.rs

@@ -1,5 +1,5 @@
+use ff::{PrimeField, PrimeFieldRepr};
 use pairing::bls12_381::Bls12;
-use pairing::{PrimeField, PrimeFieldRepr};
 use rand::{OsRng, Rng};
 use sapling_crypto::jubjub::{edwards, JubjubBls12};
 use sapling_crypto::primitives::{Diversifier, ViewingKey};

librustzcash/src/tests/key_components.rs

@@ -1,4 +1,5 @@
-use pairing::{bls12_381::Bls12, PrimeField, PrimeFieldRepr};
+use ff::{PrimeField, PrimeFieldRepr};
+use pairing::bls12_381::Bls12;
 use sapling_crypto::{
     jubjub::{fs::FsRepr, FixedGenerators, JubjubEngine, JubjubParams},
     primitives::{Diversifier, ProofGenerationKey},

librustzcash/src/tests/signatures.rs

@@ -1,4 +1,5 @@
-use pairing::{bls12_381::Bls12, PrimeField, PrimeFieldRepr};
+use ff::{PrimeField, PrimeFieldRepr};
+use pairing::bls12_381::Bls12;
 use sapling_crypto::{
     jubjub::{FixedGenerators, JubjubEngine},
     redjubjub::{PrivateKey, PublicKey, Signature},