janitor: manual cargo clippy fixes

Semi-random selection of clippy topics that were
uncontroversial before:-).

Leaves about 84 deduplicated issues in the code base
according to clippy from nightly.

internal/compiler/diagnostics.rs

@@ -156,7 +156,7 @@ impl SourceFileInner {
     }
 
     pub fn version(&self) -> SourceFileVersion {
-        self.version.clone()
+        self.version
     }
 }