Smarter logic for finding a parse synch point.

Does a half to a fifth the work in normal cases; don't notice the speedup,
but makes  more breathing room for other extensions.

Speeds terrible cases by at least a factor of 10. "Terrible" == e.g. you put
""" at the start of Tkinter.py, undo it, zoom to the bottom, and start
typing in code.  Used to take about 8 seconds for ENTER to respond, now some
large fraction of a second.  The new code gets indented correctly, despite
that it all remains "string colored" until the colorizer catches up (after
which, ENTER appears instantaneous again).