asymptote: simplify string construction

Preserve the exact content of the string while using a simpler
construction. I think ~a (display-style) might be redundant on strings,
but without contracts the input might be non-strings, so keep it.

Even simpler would be (apply ~a #:separator "\n" s strs), but that
changes the hash because it misses the final newline. An uncommitted
version used (string-join (map ~a (cons s strs)) "\n" #:after-last "\n")
for a similar effect.

asymptote.rkt

@@ -8,9 +8,7 @@
 
 (define (asymptote #:cache [cache? #t] s . strs)
   (define single-str
-    (with-output-to-string
-     (lambda () (for ([str (in-list `(,s . ,strs))])
-                  (displayln str)))))
+    (string-append (apply ~a #:separator "\n" s strs) "\n"))
   (if cache?
       ;; cache:
       (let* ([asymptote-dir "asymptote-images"]
@@ -54,4 +52,4 @@
          (λ ()
            ;(with-output-to-string
            (system (format "asy -v -f png -o ~a" tmp-file))))
-        tmp-file))) ; HTML png PDF pdf
+        tmp-file))) ; HTML png PDF pdf