(web http): keys are always symbols

* module/web/http.scm (parse-media-type): Parse media types as symbols.
  (parse-key-value-list, parse-param-component, parse-param-list):
  Change kons to val-parser. Always parse keys as symbols, and always
  either cons, if there is a val, or just have the key, if there is no
  val.  Easier to explain and just as correct.
  (declare-param-list-header!, declare-key-value-list-header!): Adapt to
  key-list and param-list kons change.
  ("Cache-Control", "Pragma", "Transfer-Encoding", "Accept", "Expect")
  ("TE"): Likewise, adapt.
  ("Content-Type"): Param keys are symbols.

examples/web/debug-sxml.scm

@@ -30,8 +30,8 @@
                   (status 200)
                   (title default-title)
                   (doctype html5-doctype)
-                  (content-type-params '(("charset" . "utf-8")))
-                  (content-type "text/html")
+                  (content-type-params '((charset . "utf-8")))
+                  (content-type 'text/html)
                   (extra-headers '())
                   (sxml (and body (templatize #:title title #:body body))))
   (values (build-response

examples/web/hello.scm

@@ -23,7 +23,7 @@
 ;; for us with a 200 OK status.
 ;;
 (define (handler request body)
-  (values '((content-type . ("text/plain")))
+  (values '((content-type . (text/plain)))
           "Hello, World!"))
 
 (run-server handler)