criteria: add infrastructure

src/criteria/crit_graph.rs

@@ -0,0 +1,16 @@
+mod crit_downstream;
+mod crit_middle;
+mod crit_root;
+mod crit_target;
+mod crit_upstream;
+
+pub use {
+    crit_downstream::{CritDownstream, CritDownstreamData},
+    crit_middle::{CritMiddle, CritMiddleCriterion},
+    crit_root::{
+        CritFixedRootCriterion, CritFixedRootCriterionBase, CritRoot, CritRootCriterion,
+        CritRootFixed,
+    },
+    crit_target::{CritMgr, CritTarget, CritTargetOwner, WeakCritTargetOwner},
+    crit_upstream::{CritUpstreamData, CritUpstreamNode},
+};