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

juno_samples/matmul/src/cpu.sch

@@ -46,16 +46,15 @@ associative(matmul@outer);
 // Parallelize by computing output array as 16 chunks
 let par = matmul@outer \ matmul@inner;
 fork-tile![4](par);
-let res = fork-reshape[[1, 3], [0], [2]](par); let outer = res.0; let inner = res.1;
+let (outer, inner, _) = fork-reshape[[1, 3], [0], [2]](par);
 parallelize!(outer \ inner);
 
 let body = outline(inner);
 cpu(body);
 
 // Tile for cache, assuming 64B cache lines
-fork-split(body);
 fork-tile![16](body);
-let res = fork-reshape[[0, 2, 4, 1, 3], [5]](body); let outer = res.0; let inner = res.1;
+let (outer, inner) = fork-reshape[[0, 2, 4, 1, 3], [5]](body);
 
 reduce-slf(inner);
 unforkify!(body);