`func MapToG2(u fptower.E2) G2Affine` takes an `internal/` object as input

Thanks to @jannotti for reporting;

Since there is no way for an external package to declare a `fptower.E2`, user must do 

```go
fp2 := bn254.G2Affine{}.X // no way to declare an fptower.E2
```

