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`