Revert mmr/partial.rs leaves() function to const LEAVES once P3 constructors are const

[huitseeker]

In miden-crypto/src/merkle/mmr/partial.rs at line 643, the const LEAVES array was changed to fn leaves() to work around the fact that P3's int_to_node function is not const.

Change Made

-    const LEAVES: [Word; 7] = [
-        int_to_node(0),
-        int_to_node(1),
-        int_to_node(2),
-        int_to_node(3),
-        int_to_node(4),
-        int_to_node(5),
-        int_to_node(6),
-    ];
+    fn leaves() -> [Word; 7] {
+        [
+            int_to_node(0),
+            int_to_node(1),
+            int_to_node(2),
+            int_to_node(3),
+            int_to_node(4),
+            int_to_node(5),
+            int_to_node(6),
+        ]
+    }

Action Required

Once upstream Plonky3 releases our changes that make their constructors const, revert this function back to a const array.

Part of #724