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

test/contracts/using_namespace_ambiguous_name.aes

@@ -0,0 +1,13 @@
+namespace Xa =
+  function f() = 1
+
+namespace Xb =
+  function f() = 2
+
+contract Cntr =
+  using Xa as A
+  using Xb as A
+
+  type state = int
+
+  entrypoint init() = A.f()