small refactor and cleanup

tests/io_read_stdin.sl

@@ -0,0 +1,13 @@
+import ../stdlib/stdlib.sl
+import ../stdlib/io.sl
+
+: main
+    1024
+    read_stdin   # returns (addr len)
+    dup 0 > if
+        write_buf
+        0 exit
+    end
+    "read_stdin failed" puts
+    exit
+;