Add dependency: bit-set.

We will be using bit-sets to keep track of which puzzle variables are
used in which constraints, and which constraints need to be woken up.

Cargo.toml

@@ -5,3 +5,4 @@ authors = ["David Wang <millimillenary@gmail.com>"]
 license = "MIT"
 
 [dependencies]
+bit-set = "*"

src/lib.rs

@@ -1,2 +1,4 @@
 //! This crate searches for the solutions to logic puzzles.
 //! The puzzle rules are expressed as constraints.
+
+extern crate bit_set;