Purge using namespace std

CODING_STYLE.md

@@ -51,11 +51,11 @@ To set indentation and tab width settings uniformly, the repository contains an
 ## 1. Namespaces
 
 1. No `using namespace` declarations in header files.
-2. Use `using namespace std;` in cpp files, but avoid importing namespaces from boost and others.
-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` and `std::forward`, which will otherwise cause the `check_style` step to fail.
+2. `using namespace solidity;` and other project local namespaces is fine in cpp files, and generally encouraged.
+3. Avoid `using namespace` at file level for third party libraries, such as boost, ranges, etc.
+4. All symbols should be declared in a namespace except for final applications.
+5. Use anonymous namespaces for helpers whose scope is a cpp file only.
+6. Preprocessor symbols should be prefixed with the namespace in all-caps and an underscore.
 
 Only in the header:
 ```cpp
@@ -66,16 +66,6 @@ std::tuple<float, float> meanAndSigma(std::vector<float> const& _v);
 }
 ```
 
-Only in the cpp file:
-```cpp
-#include <cassert>
-using namespace std;
-tuple<float, float> myNamespace::meanAndSigma(vector<float> const& _v)
-{
-  // ...
-}
-```
-
 ## 2. Preprocessor
 
 1. File comment is always at top, and includes: