web: Location header is URI-reference; better URI-reference support

* module/web/uri.scm (validate-uri): Add reference? keyword argument,
  for validating references.
  (build-uri): Clarify comments to indicate that the result is an
  absolute URI.
  (build-uri-reference): New interface, to build URI-references.
  (string->uri-reference): Rename from string->uri*.  Fix fragment
  parsing to not include the #.
  (string->uri): Adapt to string->uri-reference name change.

* module/web/request.scm (request-absolute-uri): Add default-scheme
  optional argument.  Use it if the request-uri has no scheme, or
  error.

* module/web/http.scm (write-uri): Reflow to use "when".  Fix writing of
  URI-reference instances.
  (declare-uri-reference-header!): Rename from
  declare-relative-uri-header!.  Use string->uri-reference.
  ("Location"): Declare as a URI-reference header, as per RFC 7231.

* module/web/client.scm (open-socket-for-uri): Handle the case in which
  there is no URI scheme.

* test-suite/tests/web-http.test:
* test-suite/tests/web-uri.test: Add tests.

module/web/client.scm

@@ -74,7 +74,8 @@
       (delete-duplicates
        (getaddrinfo (uri-host uri)
                     (cond (port => number->string)
-                          (else (symbol->string (uri-scheme uri))))
+                          ((uri-scheme uri) => symbol->string)
+                          (else (error "Not an absolute URI" uri)))
                     (if port
                         AI_NUMERICSERV
                         0))