Adjusted check syntax to properly deal with different identifiers
that have overlapping ranges in the editor; also got rid of the
id-set stuff

Getting rid of the id-set information that was computed means that now
the mouse-over green bubbles, the "jump to next binding occurrence"/
"jump to bound occurrence" keybindings/menu items, and the renaming
are all being computed from the arrows information as needed, instead
of building up sets as check syntax collects information. This may
change the way Check Syntax behaves in some cases; so far the only
example I've found has been strange and are arguably for the
better. Specifically, this program

  (define-syntax-rule (m x) (λ (x) x)) (m z)

no longer draws a green bubble when you mouse over the "z", since
there are no arrows (the only arrow that might have been drawn is
discarded since its start and end points are the same place).

This speeds up the "analyze the expanded code" phase of check syntax,
making it approximately 1.6x faster than before (going from about 31
seconds to about 19 seconds for this phase for the
drracket/private/unit.rkt file (on my machine)). Also, the replay
phase is probably a bit faster now, tho, too: there were 1.07x fewer
elements to process in the trace that comes back from online check
syntax now for that same file (33063 to 30842)

Note that this is only that one phase: this doesn't count the time to
actually expand the program (the dark blue bubble phase) nor the time
to send the results between places, nor the time to replay the
collected information (the light purple bubble phase).