Add include directive

Add an include directive to include namespaces into a contract. Only allowed at the top level.

To allow includes, either call through aeso_compiler:file or set the option `allow_include` (and add `include_path`(s)).

test/contracts/included2.aes

@@ -0,0 +1,5 @@
+namespace Included2a =
+  function bar() = 43
+
+namespace Included2b =
+  function foo() = 44