first steps for the 'export' feature

compiler/ast.nim

@@ -173,7 +173,9 @@ type
     nkStmtList,           # a list of statements
     nkImportStmt,         # an import statement
     nkFromStmt,           # a from * import statement
+    nkImportExceptStmt,   # an import x except a statement
     nkIncludeStmt,        # an include statement
+    nkExportStmt,         # an export statement
     nkBindStmt,           # a bind statement
     nkMixinStmt,          # a mixin statement
     nkCommentStmt,        # a comment statement