[wpimath] Move math functionality into new wpimath library (#2629)

The wpimath library is a new library designed to separate the reusable math functionality
from the common utility library (wpiutil) and the hardware-dependent library (wpilibc/j).

Package names / include file names were NOT changed to minimize breakage.  In a future year
it would be good to revamp these for a more uniform user experience and to reduce the risk
of accidental naming conflicts.

While theoretically all of this functionality could be placed into wpiutil, several pieces
of this library (e.g. DARE) are very time-consuming to compile, so it's nice to avoid this
expense for users who only want cscore or ntcore.  It also allows for easy future separation
of build tasks vs number of workers on memory-constrained machines.

This moves the following functionality from wpiutil into wpimath:
- Eigen
- ejml
- Drake
- DARE
- wpiutil.math package (Matrix etc)
- units

And the following functionality from wpilibc/j into wpimath:
- Geometry
- Kinematics
- Spline
- Trajectory
- LinearFilter
- MedianFilter
- Feed-forward controllers

wpimath/src/main/native/include/frc/geometry/Twist2d.h

@@ -0,0 +1,58 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <units/angle.h>
+#include <units/length.h>
+#include <units/math.h>
+
+namespace frc {
+/**
+ * A change in distance along arc since the last pose update. We can use ideas
+ * from differential calculus to create new Pose2ds from a Twist2d and vise
+ * versa.
+ *
+ * A Twist can be used to represent a difference between two poses.
+ */
+struct Twist2d {
+  /**
+   * Linear "dx" component
+   */
+  units::meter_t dx = 0_m;
+
+  /**
+   * Linear "dy" component
+   */
+  units::meter_t dy = 0_m;
+
+  /**
+   * Angular "dtheta" component (radians)
+   */
+  units::radian_t dtheta = 0_rad;
+
+  /**
+   * Checks equality between this Twist2d and another object.
+   *
+   * @param other The other object.
+   * @return Whether the two objects are equal.
+   */
+  bool operator==(const Twist2d& other) const {
+    return units::math::abs(dx - other.dx) < 1E-9_m &&
+           units::math::abs(dy - other.dy) < 1E-9_m &&
+           units::math::abs(dtheta - other.dtheta) < 1E-9_rad;
+  }
+
+  /**
+   * Checks inequality between this Twist2d and another object.
+   *
+   * @param other The other object.
+   * @return Whether the two objects are not equal.
+   */
+  bool operator!=(const Twist2d& other) const { return !operator==(other); }
+};
+}  // namespace frc