Update loom dependency (#105)

The old version depends on a pre 0.7 generator version that has the
RUSTSEC-2020-0151 advisory issued, causing cargo deny to fail.

Author: vkrasnov

Cargo.toml

@@ -15,4 +15,4 @@ categories = ["concurrency"]
 slab = "0.4"
 
 [target.'cfg(loom)'.dependencies]
-loom = "0.4.0"
+loom = "0.5.6"