Update loom to 0.5 (#494)

Author: taiki-e

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 env:
   RUSTFLAGS: -Dwarnings
   RUST_BACKTRACE: 1
-  nightly: nightly-2020-12-17
+  nightly: nightly-2021-04-13
 
 defaults:
   run:

Cargo.toml

@@ -31,7 +31,7 @@ serde = { version = "1.0.60", optional = true, default-features = false, feature
 serde_test = "1.0"
 
 [target.'cfg(loom)'.dev-dependencies]
-loom = "0.4"
+loom = "0.5"
 
 [package.metadata.docs.rs]
 rustdoc-args = ["--cfg", "docsrs"]