Reinstate whitespace mode.

It was useful to test external package stuff with it, but I realize now that
there's probably not a huge amount of value in leaving it external.
Cope with point being non-zero out of the gate.
Editor.newBuffer -> visitFile.

It also returns the BufferView for the visitor. This allows one to easily
navigate to a file and position the point as desired. It feels a bit hacky, but
I'm not sure I'm ready to expose all the views to everyone all the time...
Nixed stale TODO.
Fixed issues with findForward/Backward.

They weren't considering the position at the end of each line.