Re-export common types in the cretonne::ir module.

Clients should not have to navigate the ir sub-modules to find commonly used
types.

src/libcretonne/ir/jumptable.rs

@@ -120,7 +120,7 @@ impl Display for JumpTableData {
 #[cfg(test)]
 mod tests {
     use super::JumpTableData;
-    use ir::entities::Ebb;
+    use ir::Ebb;
     use entity_map::EntityRef;
 
     #[test]