Add Radian.

Author: trekhleb

src/algorithms/math/radian/README.md

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+# Radian
+
+The **radian** (symbol **rad**) is the unit for measuring angles, and is the 
+standard unit of angular measure used in many areas of mathematics.
+
+The length of an arc of a unit circle is numerically equal to the measurement 
+in radians of the angle that it subtends; one radian is just under `57.3` degrees.
+
+An arc of a circle with the same length as the radius of that circle subtends an 
+angle of `1 radian`. The circumference subtends an angle of `2π radians`.
+
+![Radian](https://upload.wikimedia.org/wikipedia/commons/4/4e/Circle_radians.gif)
+
+A complete revolution is 2π radians (shown here with a circle of radius one and 
+thus circumference `2π`).
+
+![2 pi Radian](https://upload.wikimedia.org/wikipedia/commons/6/67/2pi-unrolled.gif)
+
+**Conversions**
+
+| Radians | Degrees |
+| :-----: | :-----: |
+| 0       | 0°      |
+| π/12    | 15°     |
+| π/6     | 30°     |
+| π/4     | 45°     |
+| 1       | 57.3°   |
+| π/3     | 60°     |
+| π/2     | 90°     |
+| π       | 180°    |
+| 2π      | 360°    |
+
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Radian)

src/algorithms/math/radian/__test__/degreeToRadian.test.js

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+import degreeToRadian from '../degreeToRadian';
+
+describe('degreeToRadian', () => {
+  it('should convert degree to radian', () => {
+    expect(degreeToRadian(0)).toBe(0);
+    expect(degreeToRadian(45)).toBe(Math.PI / 4);
+    expect(degreeToRadian(90)).toBe(Math.PI / 2);
+    expect(degreeToRadian(180)).toBe(Math.PI);
+    expect(degreeToRadian(270)).toBe(3 * Math.PI / 2);
+    expect(degreeToRadian(360)).toBe(2 * Math.PI);
+  });
+});

src/algorithms/math/radian/__test__/radianToDegree.test.js

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+import radianToDegree from '../radianToDegree';
+
+describe('radianToDegree', () => {
+  it('should convert radian to degree', () => {
+    expect(radianToDegree(0)).toBe(0);
+    expect(radianToDegree(Math.PI / 4)).toBe(45);
+    expect(radianToDegree(Math.PI / 2)).toBe(90);
+    expect(radianToDegree(Math.PI)).toBe(180);
+    expect(radianToDegree(3 * Math.PI / 2)).toBe(270);
+    expect(radianToDegree(2 * Math.PI)).toBe(360);
+  });
+});

src/algorithms/math/radian/degreeToRadian.js

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+/**
+ * @param {number} degree
+ * @return {number}
+ */
+export default function degreeToRadian(degree) {
+  return degree * (Math.PI / 180);
+}

src/algorithms/math/radian/radianToDegree.js

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+/**
+ * @param {number} radian
+ * @return {number}
+ */
+export default function radianToDegree(radian) {
+  return radian * (180 / Math.PI);
+}