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.aes

@@ -0,0 +1,36 @@
+include "Option.aes"
+include "Pair.aes"
+include "String.aes"
+include "Triple.aes"
+
+using Pair
+using Triple hiding [fst, snd]
+
+namespace Nsp =
+  using Option
+
+  function h() =
+    let op = Some((2, 3, 4))
+    if (is_some(op))
+      thd(force(op)) == 4
+    else
+      false
+
+contract Cntr =
+  using Nsp
+
+  entrypoint init() = ()
+
+  function f() =
+    let p = (1, 2)
+    if (h())
+      fst(p)
+    else
+      snd(p)
+
+  function g() =
+    using String for [concat]
+
+    let s1 = "abc"
+    let s2 = "def"
+    concat(s1, s2)

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()

test/contracts/using_namespace_hidden_parts.aes

@@ -0,0 +1,8 @@
+namespace Nsp =
+  function f() = 1
+  function g() = 2
+
+contract Cntr =
+  using Nsp for [f]
+
+  entrypoint init() = g()

test/contracts/using_namespace_undefined.aes

@@ -0,0 +1,4 @@
+contract C =
+  using MyUndefinedNamespace
+
+  entrypoint init() = ()

test/contracts/using_namespace_undefined_parts.aes

@@ -0,0 +1,7 @@
+namespace Nsp =
+  function f() = 1
+
+contract Cntr =
+  using Nsp for [a]
+
+  entrypoint init() = f()

test/contracts/using_namespace_wrong_scope.aes

@@ -0,0 +1,21 @@
+namespace Nsp1 =
+  function f() = 1
+
+namespace Nsp2 =
+  using Nsp1
+
+  function g() = 1
+
+contract Cntr =
+  using Nsp2
+
+  type state = int
+
+  function x() =
+    using Nsp1
+    f()
+
+  function y() =
+    f()
+
+  entrypoint init() = f()