Update zcash_primitives crate to use ff crate

Cargo.lock

@@ -435,6 +435,7 @@ version = "0.0.0"
 dependencies = [
  "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)",
  "pairing 0.14.2",
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",

zcash_primitives/Cargo.toml

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

zcash_primitives/src/lib.rs

@@ -3,6 +3,7 @@ extern crate lazy_static;
 
 extern crate blake2_rfc;
 extern crate byteorder;
+extern crate ff;
 extern crate pairing;
 extern crate rand;
 extern crate sapling_crypto;

zcash_primitives/src/transaction/components.rs

@@ -1,8 +1,6 @@
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
-use pairing::{
-    bls12_381::{Bls12, Fr, FrRepr},
-    PrimeField, PrimeFieldRepr,
-};
+use ff::{PrimeField, PrimeFieldRepr};
+use pairing::bls12_381::{Bls12, Fr, FrRepr};
 use sapling_crypto::{
     jubjub::{edwards, Unknown},
     redjubjub::{PublicKey, Signature},

zcash_primitives/src/transaction/sighash.rs

@@ -1,6 +1,6 @@
 use blake2_rfc::blake2b::Blake2b;
 use byteorder::{LittleEndian, WriteBytesExt};
-use pairing::{PrimeField, PrimeFieldRepr};
+use ff::{PrimeField, PrimeFieldRepr};
 
 use super::{
     components::{Amount, Script, TxOut},