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/include.aes

@@ -0,0 +1,10 @@
+include "included.aes"
+include "../contracts/included2.aes"
+
+contract Include =
+  // include "maps.aes"
+  function foo() =
+    Included.foo() < Included2a.bar()
+
+  function bar() =
+    Included2b.foo() > Included.foo()

test/contracts/included.aes

@@ -0,0 +1,2 @@
+namespace Included =
+  function foo() = 42

test/contracts/included2.aes

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