Merge pull request #13554 from ethereum/code-style-rule-for-std-qualifier

Add code style rule for std:: qualifier

Author: nikola-matic

CODING_STYLE.md

@@ -55,6 +55,7 @@ To set indentation and tab width settings uniformly, the repository contains an
 3. All symbols should be declared in a namespace except for final applications.
 4. Use anonymous namespaces for helpers whose scope is a cpp file only.
 5. Preprocessor symbols should be prefixed with the namespace in all-caps and an underscore.
+6. Do not use `std::` qualifier in cpp files (see 2.), except for `std::move`, which will otherwise cause the `check_style` step to fail.
 
 Only in the header:
 ```cpp