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.

Cargo.lock

@@ -23,6 +23,7 @@ version = "0.12.0"
 dependencies = [
  "chrono",
  "dhat",
+ "dlx",
  "itertools",
  "pico-args",
  "rayon",
@@ -45,6 +46,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "anyhow"
+version = "1.0.100"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
+
 [[package]]
 name = "autocfg"
 version = "1.1.0"
@@ -154,6 +161,16 @@ dependencies = [
  "thousands",
 ]
 
+[[package]]
+name = "dlx"
+version = "0.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "52b53f18c5d4079f2e73765df0516f171cc4d7291a3fdc3a5cbb815178255c1a"
+dependencies = [
+ "anyhow",
+ "num-traits",
+]
+
 [[package]]
 name = "either"
 version = "1.15.0"