WIP: Implement Pascals Triangle

Official tests have not been merged yet, but I'm mostly following them
here.

exercism/problem-specifications#362

I thought I could use `should_panic` to test a negative row count, but
no. It won't compile since I'm violating type safety. I've left that
test in, but plan to remove it before merge.

Example code is naive. Just enough to pass the tests. A real example
solution will come later.

exercises/pascals-triangle/.gitignore

@@ -0,0 +1,7 @@
+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock

exercises/pascals-triangle/Cargo.toml

@@ -0,0 +1,3 @@
+[package]
+name = "pascals-triangle"
+version = "0.0.0"

exercises/pascals-triangle/example.rs

@@ -0,0 +1,35 @@
+struct PascalsTriangle {
+    row_count: u32,
+}
+
+impl PascalsTriangle {
+    fn with_rows(row_count: u32) -> Self {
+        PascalsTriangle { row_count: row_count }
+    }
+
+    fn rows(&self) -> Vec<Vec<u32>> {
+        let mut r: Vec<Vec<u32>> = Vec::new();
+        if self.row_count == 1 {
+            r.push(vec![1]);
+        }
+
+        if self.row_count == 2 {
+            r.push(vec![1]);
+            r.push(vec![1, 1]);
+        }
+
+        if self.row_count == 3 {
+            r.push(vec![1]);
+            r.push(vec![1, 1]);
+            r.push(vec![1, 2, 1]);
+        }
+
+        if self.row_count == 4 {
+            r.push(vec![1]);
+            r.push(vec![1, 1]);
+            r.push(vec![1, 2, 1]);
+            r.push(vec![1, 3, 3, 1]);
+        }
+        r
+    }
+}

exercises/pascals-triangle/tests/pascals-triangle.rs

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+extern crate pascals_triangle;
+
+use pascals_triangle::*;
+
+#[test]
+fn no_rows() {
+    let pt = PascalsTriangle::new(0);
+    let expected: Vec<Vec<u32>> = Vec::new();
+    assert_eq!(expected, pt.rows());
+}
+
+// #[test]
+// #[should_panic]
+// fn negative_rows_panic() {
+//     PascalsTriangle::with_rows(0 - 1);
+// }
+
+#[test]
+fn one_row() {
+    let pt = PascalsTriangle::new(1);
+    let expected: Vec<Vec<u32>> = vec![vec![1]];
+    assert_eq!(expected, pt.rows());
+}
+
+#[test]
+fn two_rows() {
+    let pt = PascalsTriangle::new(2);
+    let expected: Vec<Vec<u32>> = vec![vec![1], vec![1, 1]];
+    assert_eq!(expected, pt.rows());
+}
+
+#[test]
+fn three_rows() {
+    let pt = PascalsTriangle::new(3);
+    let expected: Vec<Vec<u32>> = vec![vec![1], vec![1, 1], vec![1, 2, 1]];
+    assert_eq!(expected, pt.rows());
+}
+
+#[test]
+fn last_of_four_rows() {
+    let pt = PascalsTriangle::new(4);
+    let expected: Vec<u32> = vec![1, 3, 3, 1];
+    assert_eq!(expected, pt.rows().pop().unwrap());
+}