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)).

src/aeso_scan.erl

@@ -36,7 +36,7 @@ lexer() ->
         , {"\\*/",        pop(skip())}
         , {"[^/*]+|[/*]", skip()} ],
 
-    Keywords = ["contract", "import", "let", "rec", "switch", "type", "record", "datatype", "if", "elif", "else", "function",
+    Keywords = ["contract", "include", "let", "rec", "switch", "type", "record", "datatype", "if", "elif", "else", "function",
                 "stateful", "true", "false", "and", "mod", "public", "private", "indexed", "internal", "namespace"],
     KW = string:join(Keywords, "|"),