Add Day 12 solution (ft. an unused DLX library)

Turns out two simple heuristics solve the whole problem in 4ms:
1. Does the area fit? No → invalid
2. Are there enough 3x3 spaces for all presents? Yes → valid

But hey, at least we learned how to use Dancing Links for
a problem that didn't need it. The dlx crate thanks us for
the download stats.

data/examples/12.txt

@@ -0,0 +1,34 @@
+0:
+###
+##.
+##.
+
+1:
+###
+##.
+.##
+
+2:
+.##
+###
+##.
+
+3:
+##.
+###
+##.
+
+4:
+###
+#..
+###
+
+5:
+###
+.#.
+###
+
+4x4: 0 0 0 0 2 0
+12x5: 1 0 1 0 2 2
+12x5: 1 0 1 0 3 2
+