2564 - simpler #words mgmt in parenthesize

Inspired indirectly by https://cuddle.googlecode.com/hg/talk/lex.html
2565 - clarify that emit is the key operation
2566

Still trying to come up with a way to show the key fact:
  emit all incoming tokens, but *also* emit parens under these conditions..
2567
2568
2569 - now exit conditions are much simpler
2570 - now start simplifying the emit phase
2571
2572
2573
2574 - being safe with emitAll simplifies the logic

But at some cost in verbosity.
2575
2576

Cluster parenAtStartOfLine uses closer together.
2577
2578
2579

We always call emitAll then emit; automate that.
2580 - done simplifying parenthesize

Inner loop is now 25% shorter (60 vs 45 lines).
More pertinent: exit logic is separate from emit logic.
And buffering logic is squeezed into a region of 4 lines.
2581