Changes the implementation of highlight-range so that it
only recomputes all of the new locations from the positions
when on-reflow is called (otherwise only computing the
relevant ones) and make the on-reflow callback chop itself
up, in case there are lots of highlighted ranges to avoid
tying up the event loop.
Changes searching so that it doesn't neccessarily compute
the entire search results in a single event callback
(but also make it start the computation more aggressively)
Overall, this changes the strategy from one that, for any potentially
long-running callback, just tried to push it off into the future, into
a strategy that tries to avoid long-running callbacks by breaking the
work up into chunks, but starting the first chunk immediately (in a
low-priority callback).
Also, misc other changes to make this work better and generally clean
things up.
