sum_tree: Replace rayon with futures (#41586)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Kate <kate@zed.dev>

crates/sum_tree/Cargo.toml

@@ -15,10 +15,12 @@ doctest = false
 
 [dependencies]
 arrayvec = "0.7.1"
-rayon.workspace = true
 log.workspace = true
+futures.workspace = true
+itertools.workspace = true
 
 [dev-dependencies]
 ctor.workspace = true
 rand.workspace = true
 zlog.workspace = true
+pollster = "0.4.0"