Vendor dependencies

Let's see how I like this workflow.

vendor/cassowary/tests/removal.rs

@@ -0,0 +1,30 @@
+extern crate cassowary;
+
+use cassowary::{Variable, Solver, Constraint};
+use cassowary::WeightedRelation::*;
+use cassowary::strength::*;
+
+mod common;
+
+use common::new_values;
+
+#[test]
+fn remove_constraint() {
+    let (value_of, update_values) = new_values();
+
+    let mut solver = Solver::new();
+
+    let val = Variable::new();
+
+    let constraint: Constraint = val | EQ(REQUIRED) | 100.0;
+    solver.add_constraint(constraint.clone()).unwrap();
+    update_values(solver.fetch_changes());
+
+    assert_eq!(value_of(val), 100.0);
+
+    solver.remove_constraint(&constraint).unwrap();
+    solver.add_constraint(val | EQ(REQUIRED) | 0.0).unwrap();
+    update_values(solver.fetch_changes());
+
+    assert_eq!(value_of(val), 0.0);
+}