Additional serializer fixes.

* Removed "\n" emitting from p-handlers so this can be rerouted
  via the separator callback.
* Added an 'emitsNL' flag when a handler needs to emit a newline
  unconditionally -- this is then routed through the emitNLs
  callback.
* All separators between nodes are now emitted via a single
  callback -- rather than one for endsLine for prev-node and
  startsLine for current-node.  This is done by routing all
  separator emits via emitSepChunk and not emitting buffered
  separators from emitSeparators.  Instead, when a handler starts
  a line, this sep is merged with the buffered separator and
  emitted.
* TODO: br emits an id('\n'), but this is probably not a separator.
* No change in parser tests.

Change-Id: I0de931cf663990196e2d8a9672cc055659031d2b
Merge "Additional serializer fixes."