Implement Luhn

Closes exercism#237

This mostly follows the current test suite
(exercism/problem-specifications#491), with one change.

The final canonical test adds an alphabetical character to a known
invalid string and then asserts that it's still invalid.

Instead what the test should do is alter a known-valid string and show
that it is now invalid.

That's what I've done here. I'll also propose the change to canonical
test suite.

exercises/luhn/.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/luhn/Cargo.toml

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

exercises/luhn/example.rs

@@ -0,0 +1,14 @@
+pub fn is_valid(candidate: &str) -> bool {
+    if candidate.chars().any(|c| c.is_alphabetic()) || candidate.chars().count() == 1 {
+        return false;
+    }
+
+    candidate.chars()
+        .filter(|c| c.is_numeric())
+        .map(|c| c.to_digit(10).unwrap())
+        .rev()
+        .enumerate()
+        .map(|(index, digit)| if index % 2 == 0 { digit } else { digit * 2 })
+        .map(|digit| if digit > 10 { digit - 9 } else { digit })
+        .sum::<u32>() % 10 == 0
+}

exercises/luhn/tests/luhn.rs

@@ -0,0 +1,33 @@
+extern crate luhn;
+
+use luhn::*;
+
+#[test]
+fn single_digit_string_is_invalid() {
+    assert!(!is_valid("1"));
+}
+
+#[test]
+fn single_zero_string_is_invalid() {
+    assert!(!is_valid("0"));
+}
+
+#[test]
+fn valid_canadian_sin_is_valid() {
+    assert!(is_valid("046 454 286"));
+}
+
+#[test]
+fn invalid_canadian_sin_is_invalid() {
+    assert!(!is_valid("046 454 287"));
+}
+
+#[test]
+fn invalid_credit_card_is_invalid() {
+    assert!(!is_valid("8273 1232 7352 0569"));
+}
+
+#[test]
+fn strings_that_contain_non_digits_are_invalid() {
+    assert!(!is_valid("046a 454 286"));
+}