Place Luhn before Largest Series Product

I don't think my solution is that far from what people will do. The
[existing crate](https://lunemec.github.io/rust-luhn/src/luhn2/src/lib.rs.html)
uses another crate to handle digits > 10 after doubling. If people go
down that path their solutions will vary a bit from mine, but not by a
bunch.

So the solution mostly requires a good handle on

- Converting a string to digits
- Working with iteration

The conversion of a string to digits is also part of Largest Series
Product, so we could put the exercise to right before LSP to reinforce
that concept.

Author: whit0694

config.json

@@ -122,6 +122,15 @@
         "match"
       ]
     },
+    {
+      "slug": "luhn",
+      "difficulty": 1,
+      "topics": [
+        "str to digits",
+        "iterators",
+        "higher-order functions",
+      ]
+    },
     {
       "slug": "largest-series-product",
       "difficulty": 1,