Test case.  New itemL, itemR, moveTo and iterator methods.
Fix insertL and iterator.
Make Iterator of definite size.
New divide method.
Require assertions to be enabled for testing.
Test insertR.
Experimenting with GapBranches.  Appears to justify having separate data
structure for branch level.
Remove GapBuffer.
Introduce focus.
insertR including divide works.
Tidy up some left/right code.
Fix iteration bugs.