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
```

types/types.go

@@ -6,6 +6,8 @@ import (
 	bn256 "github.com/ethereum/go-ethereum/crypto/bn256/cloudflare"
 )
 
+var Q, _ = new(big.Int).SetString("21888242871839275222246405745257275088696311157297823662689037894645226208583", 10)
+
 // R is the mod of the finite field
 var R, _ = new(big.Int).SetString("21888242871839275222246405745257275088548364400416034343698204186575808495617", 10)