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

docs/sophia_features.md

@@ -248,6 +248,66 @@ Functions in namespaces have access to the same environment (including the
 with the exception of `state`, `put` and `Chain.event` since these are
 dependent on the specific state and event types of the contract.
 
+To avoid mentioning the namespace every time it is used, Sophia allows
+including the namespace in the current scope with the `using` keyword:
+```
+include "Pair.aes"
+using Pair
+contract C =
+  type state = int
+  entrypoint init() =
+    let p = (1, 2)
+    fst(p)  // this is the same as Pair.fst(p)
+```
+
+It is also possible to make an alias for the namespace with the `as` keyword:
+```
+include "Pair.aes"
+contract C =
+  using Pair as P
+  type state = int
+  entrypoint init() =
+    let p = (1, 2)
+    P.fst(p)  // this is the same as Pair.fst(p)
+```
+
+Having the same alias for multiple namespaces is possible and it allows
+referening functions that are defined in different namespaces and have
+different names with the same alias:
+```
+namespace Xa = function f() = 1
+namespace Xb = function g() = 2
+contract Cntr =
+  using Xa as A
+  using Xb as A
+  type state = int
+  entrypoint init() = A.f() + A.g()
+```
+
+Note that using functions with the same name would result in an ambiguous name
+error:
+```
+namespace Xa = function f() = 1
+namespace Xb = function f() = 2
+contract Cntr =
+  using Xa as A
+  using Xb as A
+  type state = int
+
+  // the next line has an error because f is defined in both Xa and Xb
+  entrypoint init() = A.f()
+```
+
+Importing specific parts of a namespace or hiding these parts can also be
+done like this:
+```
+using Pair for [fst, snd]       // this will only import fst and snd
+using Triple hiding [fst, snd]  // this will import everything except for fst and snd
+```
+
+Note that it is possible to use a namespace in the top level of the file, in the
+contract level, namespace level, or in the function level.
+
 ## Splitting code over multiple files
 
 Code from another file can be included in a contract using an `include`