Centralize API for working set to a single struct

Rather than allow addressing tasks either by working set ID or uuid,
with attendant performance issues, this moves the API for the working
set to a single struct that just serves as a 1-1 mapping of indexes to
UUIDs.  It's up to the caller to use this information.

cli/src/invocation/cmd/add.rs

@@ -33,7 +33,11 @@ mod test {
         execute(&mut w, &mut replica, modification).unwrap();
 
         // check that the task appeared..
-        let task = replica.get_working_set_task(1).unwrap().unwrap();
+        let working_set = replica.working_set().unwrap();
+        let task = replica
+            .get_task(working_set.by_index(1).unwrap())
+            .unwrap()
+            .unwrap();
         assert_eq!(task.get_description(), "my description");
         assert_eq!(task.get_status(), Status::Pending);
 

cli/src/invocation/cmd/info.rs

@@ -12,6 +12,8 @@ pub(crate) fn execute<W: WriteColor>(
     filter: Filter,
     debug: bool,
 ) -> Fallible<()> {
+    let working_set = replica.working_set()?;
+
     for task in filtered_tasks(replica, &filter)? {
         let uuid = task.get_uuid();
 
@@ -24,7 +26,7 @@ pub(crate) fn execute<W: WriteColor>(
             }
         } else {
             t.add_row(row![b->"Uuid", uuid]);
-            if let Some(i) = replica.get_working_set_index(uuid)? {
+            if let Some(i) = working_set.by_uuid(uuid) {
                 t.add_row(row![b->"Id", i]);
             }
             t.add_row(row![b->"Description", task.get_description()]);