Add constant for tau to Math and StrictMath

Summary

Add a constant for tau, a value equal to pi*2.

Problem

Many mathematical formulas use "2*pi" as a value, making giving that quantity a distinct name useful.

Solution

Add a constant for tau to java.lang.Math and java.lang.StrictMath, alongside the constants for pi.

Specification

In both java.lang.Math and java.lang.StrictMath add:

/**
 * The {@code double} value that is closer than any other to
 * <i>tau</i> (&tau;), the ratio of the circumference of a circle
 * to its radius.
 *
 * @apiNote
 * The value of <i>pi</i> is one half that of <i>tau</i>; in other
 * words, <i>tau</i> is double <i>pi</i> .
 *
 * @since 19
 */
public static final double TAU = 2.0 * PI;