Add ProvingKey binary parser

The time on the parsing doesn't improve, as the data from the binary
file needs to be converted to `affine` representation, and then parsed
into the `bn256.G1` & `bn256.G2` formats (Montgomery). But the size of
the ProvingKey files in binary is much smaller, so will be better
handled by the smarthpones.

- Parse time benchmarks:
```
BenchmarkParsePk/Parse_Pk_bin_circuit1k-4         	       2	 563729994 ns/op
BenchmarkParsePk/Parse_Pk_json_circuit1k-4        	  635641	      1941 ns/op
BenchmarkParsePk/Parse_Pk_bin_circuit5k-4         	       1	2637941658 ns/op
BenchmarkParsePk/Parse_Pk_json_circuit5k-4        	       1	2986560185 ns/op
BenchmarkParsePk/Parse_Pk_bin_circuit10k-4        	       1	5337639150 ns/op
BenchmarkParsePk/Parse_Pk_json_circuit10k-4       	       1	6149568824 ns/op
BenchmarkParsePk/Parse_Pk_bin_circuit20k-4        	       1	10533654623 ns/op
BenchmarkParsePk/Parse_Pk_json_circuit20k-4       	       1	11657326851 ns/op
```

- Size of ProvingKey file for a circuit of 50k constraints:
```
circuit 20k constraints:
10097812 bytes of proving_key.bin
29760049 bytes of proving_key.json

circuit 50k constraints:
24194964 bytes of proving_key.bin
71484081 bytes of proving_key.json
```

parsers/parsers_test.go

@@ -196,3 +196,63 @@ func TestProofSmartContractFormat(t *testing.T) {
 	assert.Equal(t, pSC, pSC2)
 
 }
+
+func testCircuitParsePkBin(t *testing.T, circuit string) {
+	pkBinFile, err := os.Open("../testdata/" + circuit + "/proving_key.bin")
+	require.Nil(t, err)
+	defer pkBinFile.Close()
+	pk, err := ParsePkBin(pkBinFile)
+	require.Nil(t, err)
+
+	pkJson, err := ioutil.ReadFile("../testdata/" + circuit + "/proving_key.json")
+	require.Nil(t, err)
+	pkJ, err := ParsePk(pkJson)
+	require.Nil(t, err)
+
+	assert.Equal(t, pkJ.NVars, pk.NVars)
+	assert.Equal(t, pkJ.NPublic, pk.NPublic)
+	assert.Equal(t, pkJ.DomainSize, pk.DomainSize)
+	assert.Equal(t, pkJ.VkAlpha1, pk.VkAlpha1)
+	assert.Equal(t, pkJ.VkBeta1, pk.VkBeta1)
+	assert.Equal(t, pkJ.VkDelta1, pk.VkDelta1)
+	assert.Equal(t, pkJ.VkDelta2, pk.VkDelta2)
+	assert.Equal(t, pkJ.PolsA, pk.PolsA)
+	assert.Equal(t, pkJ.PolsB, pk.PolsB)
+	assert.Equal(t, pkJ.A, pk.A)
+	assert.Equal(t, pkJ.B1, pk.B1)
+	assert.Equal(t, pkJ.B2, pk.B2)
+	assert.Equal(t, pkJ.C, pk.C)
+	assert.Equal(t, pkJ.HExps[:pkJ.DomainSize], pk.HExps[:pk.DomainSize]) // circom behaviour
+}
+
+func TestParsePkBin(t *testing.T) {
+	testCircuitParsePkBin(t, "circuit1k")
+	testCircuitParsePkBin(t, "circuit5k")
+}
+
+func benchmarkParsePk(b *testing.B, circuit string) {
+	pkJson, err := ioutil.ReadFile("../testdata/" + circuit + "/proving_key.json")
+	require.Nil(b, err)
+
+	pkBinFile, err := os.Open("../testdata/" + circuit + "/proving_key.bin")
+	require.Nil(b, err)
+	defer pkBinFile.Close()
+
+	b.Run("Parse Pk bin "+circuit, func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ParsePk(pkJson)
+		}
+	})
+	b.Run("Parse Pk json "+circuit, func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ParsePkBin(pkBinFile)
+		}
+	})
+}
+
+func BenchmarkParsePk(b *testing.B) {
+	benchmarkParsePk(b, "circuit1k")
+	benchmarkParsePk(b, "circuit5k")
+	// benchmarkParsePk(b, "circuit10k")
+	// benchmarkParsePk(b, "circuit20k")
+}