Optimize MiMC7 migrating from *big.Int to goff generated finite field
operations.
There is still a lot of room for optimization for MiMC7 in the way that is done internally, but will be done in the future.
Benchmarks:
Tested on a Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz, with 16GB of RAM.
- Before:
```
BenchmarkMIMC7-4 1026 1160298 ns/op
```
- After this commit:
```
BenchmarkMIMC7-4 19263 61651 ns/op
```
Optimize Poseidon migrating from *big.Int to goff generated finite field
operations.
Benchmarks:
Tested on a Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz, with 16GB of RAM.
- Before the optimizations:
```
BenchmarkPoseidon-4 470 2489678 ns/op
BenchmarkPoseidonLarge-4 476 2530568 ns/op
```
- With the optimizations of #12:
```
BenchmarkPoseidon-4 766 1550013 ns/op
BenchmarkPoseidonLarge-4 782 1547572 ns/op
```
- With the changes of this PR, where uses goff generated code instead of *big.Int:
```
BenchmarkPoseidon-4 9638 121651 ns/op
BenchmarkPoseidonLarge-4 9781 119921 ns/op
```
