Add http-post, http-put, et cetera

Add http-post, http-put, et cetera

* module/web/client.scm (ensure-uri): New helper.
  (open-socket-for-uri): Accept a URI as a string or as a URI object.
  (extend-request, sanitize-request): New helpers, like the
  corresponding functions in (web server).
  (decode-response-body): Add a reference to the HTTP/1.1 spec, and
  use (ice-9 iconv).
  (request): New helper, factoring all aspects of sending an HTTP
  request and getting a response.
  (http-get): Redefine in terms of http-get.  Deprecate the
  #:extra-headers argument in favor of #:headers.  Allow a body.  Add a
  #:streaming? argument, subsuming the functionality of http-get*.
  (http-get*): Deprecate.
  (http-head, http-post, http-put, http-delete, http-trace)
  (http-options): Define interfaces for all HTTP verbs.

* test-suite/tests/web-client.test: Add tests.

* doc/ref/web.texi: Update documentation.

Thanks to Gregory Benison for the initial patch.