more uri-related ipv6 fixes

more uri-related ipv6 fixes

* module/web/uri.scm (ipv6-regexp): IPv6 numeric addresses do not have
  brackets.  It's only in URIs that they have them.
  (ipv6-host-pat, authority-regexp, parse-authority): Refactor ipv6
  detection to fix a bug with |, and to extract IPv6 hosts from their
  brackets.  This way we can pass the uri-host directly to inet-pton.
  (uri->string): If the host contains a `:', assume it is ipv6 and add
  brackets.

* test-suite/tests/web-uri.test ("build-uri"): Adapt tests to assume
  that the address returned by uri-host and passed to build-uri #:host
  does not have brackets.