cache the computation of the tacked arrows x span

we still blow the budget on some calls to on-paint, but
instead of 800 msec for 5000 arrows, it's more like 40-60 msec

Author: rfindler

drracket-core-lib/drracket/private/syncheck/gui.rkt

@@ -1285,6 +1285,8 @@ If the namespace does not, they are colored the unbound color.
               (define tacked-arrows-to-draw '())
               (define arrows-to-draw '())
 
+              (define arrow->matching-identifiers-hash (make-hash))
+
               (for ([(arrow v) (in-hash tacked-hash-table)])
                 (when v
                   (define-values (arrow-text arrow-pos)
@@ -1293,11 +1295,22 @@ If the namespace does not, they are colored the unbound color.
                                                   (var-arrow-start-pos-left arrow))]
                       [(tail-arrow? arrow) (values (tail-arrow-from-text arrow)
                                                    (tail-arrow-from-pos arrow))]))
-                  (define-values (_binders make-identifiers-hash)
-                    (position->matching-identifiers-hash arrow-text arrow-pos arrow-pos
-                                                         #:also-look-backward-one? #f))
-                  (define-values (tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)
-                    (get-var-arrow-end-x-min-and-max (make-identifiers-hash)))
+                  (match-define (cons tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)
+                    (match (hash-ref
+                            arrow->matching-identifiers-hash
+                            (cons arrow-text arrow-pos)
+                            #f)
+                      [#f
+                       (define-values (_binders make-identifiers-hash)
+                         (position->matching-identifiers-hash arrow-text arrow-pos arrow-pos
+                                                              #:also-look-backward-one? #f))
+                       (define-values (tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)
+                         (get-var-arrow-end-x-min-and-max (make-identifiers-hash)))
+                       (hash-set! arrow->matching-identifiers-hash (cons arrow-text arrow-pos)
+                                  (cons tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max))
+                       (cons tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)]
+                      [(cons tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)
+                       (cons tacked-var-arrow-end-x-min tacked-var-arrow-end-x-max)]))
 
                   (set! tacked-arrows-to-draw
                         (cons