in a cell makes a final cell in that line
   (this showed up as occasionally missing single spaces in
   godoc-formatted declarations that fit on a single line)

2) Cleaned up tabwriter implementation a bit:
   - replaced local unicodeLen() with utf8.RuneCount()
   - instead of having 2 parallel arrays for line widths and sizes,
     have a single array of cells containing a width and size
   - factored code a bit better
   - added more comments
   - added testnames to tabwriter tests
   - added more test cases and fixed a broken test case that
     now works correctly

R=r
DELTA=279  (133 added, 62 deleted, 84 changed)
OCL=30509
CL=30514