1) Fix a problem with tabwriter.Flush: any pending text not yet
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