It's a little ugly to re-use the ec constants, maybe that should be
changed.

This also changes the opcode to panic on buffers than contain elements
greater than the curve's modulus.

It's unclear what mimc should do with a zero buffer.  Even gnark seems
unsure.  Their code says:

```
// TODO @ThomasPiellard shouldn't Sum() returns an error if there is no data?
// TODO: @Tabaie, @thomas Piellard Now sure what to make of this
/*if len(d.data) == 0 {
	d.data = make([]byte, BlockSize)
}*/
```

Make the parameters of the MIMC construction explicit.
Better document its intended use case.