Bump arc-swap to 1.6.0

1.6.0 fixes some UB in previous versions of `arc-swap`.

Cargo.toml

@@ -8,7 +8,7 @@ repository = "https://github.com/salsa-rs/salsa"
 description = "A generic framework for on-demand, incrementalized computation (experimental)"
 
 [dependencies]
-arc-swap = "1.4.0"
+arc-swap = "1.6.0"
 crossbeam-utils = { version = "0.8", default-features = false }
 dashmap = "5.3.4"
 hashlink = "0.8.0"