Update zcash_proofs crate to use ff crate

Cargo.lock

@@ -448,6 +448,7 @@ version = "0.0.0"
 dependencies = [
  "bellman 0.1.0",
  "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)",
  "pairing 0.14.2",
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "sapling-crypto 0.0.1",

zcash_proofs/Cargo.toml

@@ -8,6 +8,7 @@ authors = [
 [dependencies]
 bellman = { path = "../bellman" }
 byteorder = "1"
+ff = "0.4"
 pairing = { path = "../pairing" }
 rand = "0.4"
 sapling-crypto = { path = "../sapling-crypto" }

zcash_proofs/src/lib.rs

@@ -1,5 +1,6 @@
 extern crate bellman;
 extern crate byteorder;
+extern crate ff;
 extern crate pairing;
 extern crate rand;
 extern crate sapling_crypto;

zcash_proofs/src/sapling/prover.rs

@@ -2,10 +2,8 @@ use bellman::groth16::{
     create_random_proof, verify_proof, Parameters, PreparedVerifyingKey, Proof,
 };
 use byteorder::{LittleEndian, ReadBytesExt};
-use pairing::{
-    bls12_381::{Bls12, Fr, FrRepr},
-    Field, PrimeField, PrimeFieldRepr,
-};
+use ff::{Field, PrimeField, PrimeFieldRepr};
+use pairing::bls12_381::{Bls12, Fr, FrRepr};
 use rand::{OsRng, Rand};
 use sapling_crypto::{
     circuit::{

zcash_proofs/src/sapling/verifier.rs

@@ -1,8 +1,6 @@
 use bellman::groth16::{verify_proof, PreparedVerifyingKey, Proof};
-use pairing::{
-    bls12_381::{Bls12, Fr},
-    Field,
-};
+use ff::Field;
+use pairing::bls12_381::{Bls12, Fr};
 use sapling_crypto::{
     circuit::multipack,
     jubjub::{edwards, FixedGenerators, JubjubBls12, Unknown},