Summary
-------

Add `nthRoot()` and `nthRootAndRemainder()` methods in class `BigInteger`.

Problem
-------
The class `BigInteger` already provides methods `pow()`, `sqrt()` and `sqrtAndRemainder()` for power and square root calculation, so, for symmetry, it would be useful introducing methods to compute integer nth roots too. This feature can then be used to implement also a possible method `BigDecimal.nthRoot()` in a future release.
 
Solution
--------

Implement `nthRoot()` and `nthRootAndRemainder()` methods in class `BigInteger`.

Specification
-------------

    /**
     * Returns the integer {@code n}th root of this BigInteger. The integer
     * {@code n}th root {@code r} of the corresponding mathematical integer {@code x}
     * is defined as follows:
     * <ul>
     *   <li>if {@code x} &ge; 0, then {@code r} &ge; 0 is the largest integer such that
     *   {@code r}<sup>{@code n}</sup> &le; {@code x};
     *   <li>if {@code x} &lt; 0, then {@code r} &le; 0 is the smallest integer such that
     *   {@code r}<sup>{@code n}</sup> &ge; {@code x}.
     * </ul>
     * If the root is defined, it is equal to the value of
     * {@code x.signum()}&sdot; &lfloor;{@code |nthRoot(x, n)|}&rfloor;,
     * where {@code nthRoot(x, n)} denotes the real {@code n}th root of {@code x}
     * treated as a real.
     * Otherwise, the method throws an {@code ArithmeticException}.
     *
     * <p>Note that the magnitude of the integer {@code n}th root will be less than
     * the magnitude of the real {@code n}th root if the latter is not representable
     * as an integral value.
     *
     * @param n the root degree
     * @return the integer {@code n}th root of {@code this}
     * @throws ArithmeticException if {@code n <= 0}.
     * @throws ArithmeticException if {@code n} is even and {@code this} is negative.
     * @see #sqrt()
     * @since 26
     */
    public BigInteger nthRoot(int n)

    /**
     * Returns an array of two BigIntegers containing the integer {@code n}th root
     * {@code r} of {@code this} and its remainder {@code this - r}<sup>{@code n}</sup>,
     * respectively.
     *
     * @param n the root degree
     * @return an array of two BigIntegers with the integer {@code n}th root at
     *         offset 0 and the remainder at offset 1
     * @throws ArithmeticException if {@code n <= 0}.
     * @throws ArithmeticException if {@code n} is even and {@code this} is negative.
     * @see #sqrt()
     * @see #sqrtAndRemainder()
     * @see #nthRoot(int)
     * @since 26
     */
    public BigInteger[] nthRootAndRemainder(int n)