New class for handling infinite-precision rationals

[Fabio Romano]

The Rational class provides operations for arithmetic, rounding,
comparison, hashing, and format conversion. The toString method provides a
canonical representation of a Rational. All the calculations performed have
an exact result, except for the square and nth roots, in which the user can
specify the rounding behavior.
Link to pull request: https://github.com/openjdk/jdk/pull/11982