Some small steps in the direction of making GHC useable as a library:

  - The ErrMsg type is now richer: we keep the location info and the
    PrintUnqualified separate until the message is printed out, and
    messages have a short summary and "extra info", where the extra
    info is used for things like the context info in the typechecker
    (stuff that you don't normally want to see in a more visual setting,
    where the context is obvious because you're looking at the code).

  - hscMain now takes an extra argument of type (Messages -> IO ()),
    which says what to do with the error messages.  In normal usage,
    we just pass ErrUtils.printErrorsAndWarnings, but eg. a development
    environment will want to do something different.  The direction we
    need to head in is for hscMain to *never* do any output to
    stdout/stderr except via abstractions like this.