WIP: Implement Pascals Triangle

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

exercism/problem-specifications#362

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

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+[package]
+name = "pascals-triangle"
+version = "0.0.0"

exercises/pascals-triangle/example.rs

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+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]
+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());
+}