build: Require Guile >= 2.0.9.

* configure.ac: Bump requirement to 2.0.9.
* doc/guix.texi (Requirements): Adjust accordingly.
* README (Requirements): Likewise.
* build-aux/download.scm: Remove workaround for <http://bugs.gnu.org/13095>.
* guix/build/download.scm: Likewise.
(http-fetch)[post-2.0.7?]: Remove.  Remove conditional code for not
POST-2.0.7?.
* guix/http-client.scm:  Remove workaround for <http://bugs.gnu.org/13095>.
(http-fetch)[post-2.0.7?]: Remove.  Remove conditional code for not
POST-2.0.7?.
* guix/serialization.scm (read-latin1-string): Remove mention of 2.0.9.
* tests/nar.scm: Use (ice-9 control).
(let/ec): Remove.

tests/nar.scm

@@ -35,6 +35,7 @@
   #:use-module (srfi srfi-64)
   #:use-module (ice-9 ftw)
   #:use-module (ice-9 regex)
+  #:use-module ((ice-9 control) #:select (let/ec))
   #:use-module (ice-9 match))
 
 ;; Test the (guix nar) module.
@@ -148,17 +149,6 @@
   (string-append (dirname (search-path %load-path "pre-inst-env"))
                  "/test-nar-" (number->string (getpid))))
 
-(define-syntax-rule (let/ec k exp...)
-  ;; This one appeared in Guile 2.0.9, so provide a copy here.
-  (let ((tag (make-prompt-tag)))
-    (call-with-prompt tag
-      (lambda ()
-        (let ((k (lambda args
-                   (apply abort-to-prompt tag args))))
-          exp...))
-      (lambda (_ . args)
-        (apply values args)))))
-
 
 (test-begin "nar")