dwarflint: Add filtering of duplicate messages

dwarflint: Add filtering of duplicate messages

- we use the first string component of stream implementation of wr_message
  as a key to decide whether we've seen the message before.  Most of the
  messages use streams, is why it's like that.  When (if) we get back to
  formatting strings, that string should be even better key, because there
  will be less chance for compiler to do duplicate elimination etc.