Fixes bug 996: /*@4*/ style content in strings confuses the module renderer

Issue 996:  Refactor rendering pipeline to always pass tokens *and* marks
    If the original source code contains a comment inside a string, like:
      var x = '/*@1*/';
    it will get cajoled to:
      var x = '';
    which is not a security breach, but is incorrect. This is because
    the current pipeline uses textual substitution to keep track of
    file positions as the source passes through.

Got rid of textual substition.
Before the process looked like
  (1) Feed tokens to SourceSpanRenderer which forwards them to a
      delegate renderer along with /*@1*/ comments wherever there is an
      original file position mark.
  (2) Find /*@1*/ inside the rendered text, remove it, and build a
      mapping from output characters to filepositions from the
      original source.
  (3) Munge that list to generate the desired debug metadata.

Now the process looks like
  (1) Feed tokens to SourceSpanRenderer which forwards them to a
      delegate renderer and keeps a list of all non-comment/space
      tokens with marks inline.
  (2) Lex the rendered output using JsLexer and match stored tokens
      with rendered tokens creating a mapping from original source
      file positions to rendered source positions.
  (3) Turn that mapping into a mapping of output characters to
      original source filepositions.
  (4) Munge as before.

I also did a bit of housecleaning:
  (1) The SideBySideRenderer and TabularSideBySideRenderer were
      obsoleted by SourceSnippetRenderer.  Deleted them.
  (2) Got rid of unnecessary IOException handler to SourceSpanRenderer
      since it always renders to a StringBuilder.
  (3) Got rid of unnecessary output filename parameter since it seems
      to be unused except in test code.
  (4) Got rid of now unused breakAfterComment parameter to
      JsPrettyPrinter and friends.

Since we no longer insert comment tokens (which were subsequently
removed), the rendered output matches exactly what an unwrapped pretty
printer would do.
This invalidated the existing unit tests, so please pay special
attention to the test goldens.