derivations: Avoid uses of 'display' in 'write-derivation'.

This yields a 4% improvement on the wall-clock time of:

  guix build -e '(@@ (gnu packages libreoffice) libreoffice)' --no-grafts -d

* guix/derivations.scm (write-sequence, write-list, write-tuple): Use
'put-char' instead of 'display'.
(write-derivation): Use 'put-string' and 'put-char', and remove unused
'format' binding.

guix/derivations.scm

@@ -26,6 +26,7 @@
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:use-module (ice-9 binary-ports)
+  #:use-module ((ice-9 textual-ports) #:select (put-char put-string))
   #:use-module (rnrs bytevectors)
   #:use-module (ice-9 match)
   #:use-module (ice-9 rdelim)
@@ -561,30 +562,29 @@ things as appropriate and is thus more efficient."
     ((prefix (... ...) last)
      (for-each (lambda (item)
                  (write-item item port)
-                 (display "," port))
+                 (put-char port #\,))
                prefix)
      (write-item last port))))
 
 (define-inlinable (write-list lst write-item port)
   ;; Write LST as a derivation list to PORT, using WRITE-ITEM to write each
   ;; element.
-  (display "[" port)
+  (put-char port #\[)
   (write-sequence lst write-item port)
-  (display "]" port))
+  (put-char port #\]))
 
 (define-inlinable (write-tuple lst write-item port)
   ;; Same, but write LST as a tuple.
-  (display "(" port)
+  (put-char port #\()
   (write-sequence lst write-item port)
-  (display ")" port))
+  (put-char port #\)))
 
 (define (write-derivation drv port)
   "Write the ATerm-like serialization of DRV to PORT.  See Section 2.4 of
 Eelco Dolstra's PhD dissertation for an overview of a previous version of
 that form."
 
-  ;; Make sure we're using the faster implementation.
-  (define format simple-format)
+  ;; Use 'put-string', which does less work and is faster than 'display'.
 
   (define (write-string-list lst)
     (write-list lst write port))
@@ -605,42 +605,41 @@ that form."
   (define (write-input input port)
     (match input
       (($ <derivation-input> obj sub-drvs)
-       (display "(\"" port)
+       (put-string port "(\"")
 
        ;; 'derivation/masked-inputs' produces objects that contain a string
        ;; instead of a <derivation>, so we need to account for that.
-       (display (if (derivation? obj)
-                    (derivation-file-name obj)
-                    obj)
-                port)
-       (display "\"," port)
+       (put-string port (if (derivation? obj)
+                            (derivation-file-name obj)
+                            obj))
+       (put-string port "\",")
        (write-string-list sub-drvs)
-       (display ")" port))))
+       (put-char port #\)))))
 
   (define (write-env-var env-var port)
     (match env-var
       ((name . value)
-       (display "(" port)
+       (put-string port "(")
        (write name port)
-       (display "," port)
+       (put-string port ",")
        (write value port)
-       (display ")" port))))
+       (put-string port ")"))))
 
   ;; Assume all the lists we are writing are already sorted.
   (match drv
     (($ <derivation> outputs inputs sources
         system builder args env-vars)
-     (display "Derive(" port)
+     (put-string port "Derive(")
      (write-list outputs write-output port)
-     (display "," port)
+     (put-char port #\,)
      (write-list inputs write-input port)
-     (display "," port)
+     (put-char port #\,)
      (write-string-list sources)
      (simple-format port ",\"~a\",\"~a\"," system builder)
      (write-string-list args)
-     (display "," port)
+     (put-char port #\,)
      (write-list env-vars write-env-var port)
-     (display ")" port))))
+     (put-char port #\)))))
 
 (define derivation->bytevector
   (lambda (drv)