Adding `big(::FixedPoint)` and `rationalize(::FixedPoint)`

Conversion to `BigFloat` is already supported. However, `Base.big()` have not been overloaded.

In practical use, we should explicitly specify the `BigFloat`, but the short `big` may be useful for debugging.

Similarly, `rationalize()` have not been overloaded. Even though the name "rationalize" is longer than "Rational", I think supporting it is better for consistency.