Add parsers from bin to pk and from pk to bin.

```
BenchmarkParsePk/ParsePkJson_circuit1k-4         	  595215	      1954 ns/op
BenchmarkParsePk/ParsePkBin_circuit1k-4          	       2	 522174688 ns/op
BenchmarkParsePk/ParsePkGoBin_circuit1k-4        	  623182	      1901 ns/op
BenchmarkParsePk/ParsePkJson_circuit5k-4         	       1	2861959141 ns/op
BenchmarkParsePk/ParsePkBin_circuit5k-4          	       1	2610638932 ns/op
BenchmarkParsePk/ParsePkGoBin_circuit5k-4        	  612906	      1907 ns/op
BenchmarkParsePk/ParsePkJson_circuit10k-4        	       1	5793696755 ns/op
BenchmarkParsePk/ParsePkBin_circuit10k-4         	       1	5204251056 ns/op
BenchmarkParsePk/ParsePkGoBin_circuit10k-4       	  603478	      1913 ns/op
BenchmarkParsePk/ParsePkJson_circuit20k-4        	       1	11404022165 ns/op
BenchmarkParsePk/ParsePkBin_circuit20k-4         	       1	10392012613 ns/op
BenchmarkParsePk/ParsePkGoBin_circuit20k-4       	  620488	      1921 ns/op
```

For the `10k` constraints circuit is arround `1_368_976x` improvement.
For the `20k` constraints circuit is arround `5_409_689x` improvement.

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
```

Also adds ProofToSmartContractFormat, which returns a ProofString as the
proof.B elements swap is not a valid point for the bn256.G2 format.

Also unexports internal structs and methods of the prover package.
Also apply golint.