C++ namespace cleanup (except tests).

libsolidity/ast/ExperimentalFeatures.h

@@ -23,9 +23,7 @@
 #include <map>
 #include <set>
 
-namespace dev
-{
-namespace solidity
+namespace solidity::frontend
 {
 
 enum class ExperimentalFeature
@@ -52,4 +50,3 @@ static std::map<std::string, ExperimentalFeature> const ExperimentalFeatureNames
 };
 
 }
-}