Introduce namespace langutil in liblangutil directory.

Also:
- Use {}-style list initialisation for SourceLocation construction
- Introduce new system includes
- Changes the API of the Scanner to take source as value (with move) as opposed to as a reference

libyul/optimiser/SimplificationRules.cpp

@@ -31,6 +31,7 @@
 
 using namespace std;
 using namespace dev;
+using namespace langutil;
 using namespace dev::yul;