Add rectangles example

+[package]
+name = "rectangles"
+version = "0.1.0"
+authors = ["Georg Hopp <georg@steffers.org>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

+//
+// Examples related to structs…
+//
+// Georg Hopp <georg@steffers.org>
+//
+// Copyright © 2019 Georg Hopp
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+
+#[derive(Debug)]
+struct Rectangle {
+    width:  u32,
+    height: u32,
+}
+
+impl Rectangle {
+    fn area(&self) -> u32 {
+        self.width * self.height
+    }
+
+    fn can_hold(&self, inner: &Rectangle) -> bool {
+        self.width >= inner.width && self.height >= inner.height
+    }
+
+    fn square(size: u32) -> Rectangle {
+        Rectangle { width: size, height: size }
+    }
+}
+
+fn main() {
+    let width1  = 30;
+    let height1 = 50;
+
+    println!(
+        "The area of the rectangle ist {} square pixels.",
+        area(width1, height1));
+
+    let rect1 = (30, 50);
+
+    println!(
+        "The area of the rectangle ist {} square pixels.",
+        _area(rect1));
+
+    let rect1 = Rectangle { width: 30, height: 50 };
+
+    println!(
+        "The area of the rectangle ist {} square pixels.",
+        __area(&rect1));
+
+    println!("_rect1 is {:?}", rect1);
+
+    println!(
+        "The area of the rectangle ist {} square pixels.",
+        rect1.area());
+
+    let rect2 = Rectangle { width: 10, height: 40};
+    let rect3 = Rectangle { width: 60, height: 45};
+
+    println!("Can rect1 hold rect2? {}", rect1.can_hold(&rect2));
+    println!("Can rect1 hold rect3? {}", rect1.can_hold(&rect3));
+
+    let square1 = Rectangle::square(32);
+
+    println!("Got the square rectange: {:?}", square1);
+}
+
+fn area(width: u32, height: u32) -> u32 {
+    width * height
+}
+
+fn _area(dimensions: (u32, u32)) -> u32 {
+    let (width, height) = dimensions;
+    width * height
+}
+
+fn __area(rectangle: &Rectangle) -> u32 {
+    rectangle.width * rectangle.height
+}