Fix clippy warnings.

This commit fixes the current set of (stable) clippy warnings in the repo.

cranelift/codegen/src/result.rs

@@ -37,6 +37,6 @@ pub type CodegenResult<T> = Result<T, CodegenError>;
 
 impl From<VerifierErrors> for CodegenError {
     fn from(e: VerifierErrors) -> Self {
-        CodegenError::Verifier(e)
+        Self::Verifier(e)
     }
 }