Optimize MiMC7 migrating from *big.Int to goff

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
```

poseidon/poseidon.go

@@ -92,7 +92,6 @@ func ark(state [T]*ff.Element, c *ff.Element) {
 
 // cubic performs x^5 mod p
 // https://eprint.iacr.org/2019/458.pdf page 8
-// var five = big.NewInt(5)
 
 func cubic(a *ff.Element) {
 	a.Exp(*a, 5)