Fork reshape

Implements a fork-reshape pass in the scheduling language which can be used to coalesce a selection of fork-joins, interchange them into a specified order, and then split them into a specified series of chunks. Adds a parallel and tiled schedule for matmul that uses fork-reshape to keep the schedule quite clean.

Also adds tuples to the scheduling language and a new form of let-statements that allow destructing tuples, it's not the nicest (it's not a general pattern handler and will only cause an error if there are too many fields on the left-hand side, not too few, but it's enough to make the code that uses fork-reshape cleaner).

Includes a few other tweaks I ran into

• When adding a node in the editor, only insert an empty def_use set if the node doesn't have one yet (this is important due to cycles between nodes, and was causing issues in fork reshape since it invokes a series of passes using the same editor)
• Made fork-coalesce return the fork and join nodes when it succeeds so that they can be used in further operations.
• Modified fork coalesce's check for control flow between the forks and joins to check via uses rather than users because I was having weird issues with checking users
• Modified fork coalesce to perform everything as a single edit because I was having issues when manually scheduling matmul when using selections with coalesce.
• Modified fork split to return the ID of the new fork on a success
• Modified both fork split and coalesce to propagate parallel fork schedules when appropriate to avoid having to re-assign/re-infer schedules

hercules_opt/src/editor.rs

@@ -457,7 +457,7 @@ impl<'a, 'b> FunctionEdit<'a, 'b> {
     pub fn add_node(&mut self, node: Node) -> NodeID {
         let id = NodeID::new(self.num_node_ids());
         // Added nodes need to have an entry in the def-use map.
-        self.updated_def_use.insert(id, HashSet::new());
+        self.updated_def_use.entry(id).or_insert(HashSet::new());
         // Added nodes use other nodes, and we need to update their def-use
         // entries.
         for u in get_uses(&node).as_ref() {