* Add using namespace as to scanner and parser
* Change the alias from id() to con()
* Add using namespace to AST type inference
* Allow using namespace to appear in the top level
* Allow using namespace to appear inside functions
* Add a compiler test for using namespace
* Handle name collisions
* Implement mk_error for ambiguous_name
* Add failing test for ambiguous names
* Limit the scope of the used namespaces
* Add test for wrong scope of using namespace
* Use a single using declaration
* Split long line
* Forbid using undefined namespaces
* Add a test for using undefined namespaces
* Change the type of used_namespaces
* Add using namespace parts to scanner and parser
* Add using namespace parts to ast type inference
* Add tests for using namespace parts
* Update CHANGELOG.md
* Code cleaning
* Update the docs
* Update the docs about the same alias for multiple namespaces
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