Implement loading namespaces with the using keyword (#338)

* 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

src/aeso_scan.erl

@@ -45,7 +45,7 @@ lexer() ->
 
     Keywords = ["contract", "include", "let", "switch", "type", "record", "datatype", "if", "elif", "else", "function",
                 "stateful", "payable", "true", "false", "mod", "public", "entrypoint", "private", "indexed", "namespace",
-                "interface", "main"
+                "interface", "main", "using", "as", "for", "hiding"
                ],
     KW = string:join(Keywords, "|"),