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.

- before:
BenchmarkGenerateProof-4               1        1553842743 ns/op
For a circuit of 9094 constraints takes 7.761949512s seconds to generate the proof.

- now:
BenchmarkGenerateProof-4               1        1331576862 ns/op
For a circuit of 9094 constraints takes 5.745279126s to generate the proof.

For bigger circuits (more constraints) the difference will be bigger.

Executed on a Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz, with 16GB of RAM

For a circuit arround 22500 constraints:
- before: the proof was generated aprox in `80 seconds`
- now: the proof is generated aprox in `16 seconds`

Polynomials and ifft moved to goff (iden3/go-iden3-crypto/ff) instead of *big.Int.

Benchmarks:

- Before:
BenchmarkArithmetic/polynomialSub-4         	    2774	    441063 ns/op
BenchmarkArithmetic/polynomialMul-4         	       1	1135732757 ns/op
BenchmarkArithmetic/polynomialDiv-4         	     768	   1425192 ns/op
BenchmarkGenerateProof-4                    	       1	2844488975 ns/op

- With this commit:
BenchmarkArithmetic/polynomialSubE-4        	   23097	     54152 ns/op
BenchmarkArithmetic/polynomialMulE-4        	      25	  44914327 ns/op
BenchmarkArithmetic/polynomialDivE-4        	    8703	    132573 ns/op
BenchmarkGenerateProof-4                    	       1	1530398526 ns/op