feat: change @rational.T to pub

[illusory0x0]

@rational.T only has immutable field, change to pub is safe, only construct @rational.T is unsafe

pub struct T {
  numerator : Int64
  denominator : Int64
}

[peter-jerry-ye]

We will move this package to x. We will have a BigDecimal instead which is more practical. As of rational, we would also replace the Int64 with BigInt internally.