revert to cortex-m/-rt v0.4.0

Cargo.toml

@@ -9,12 +9,12 @@ version = "0.3.0"
 
 [dependencies]
 bare-metal = "0.2.0"
-cortex-m = "0.5.0"
+cortex-m = "0.4.0"
 vcell = "0.1.0"
 
 [dependencies.cortex-m-rt]
 optional = true
-version = "0.5.0"
+version = "0.4.0"
 
 [features]
 rt = ["cortex-m-rt/device"]
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