* module/web/http.scm: New module, declares known HTTP headers, and
their parsers and unparsers.
* test-suite/tests/web-http.test: Add test suite.
* module/Makefile.am:
* test-suite/Makefile.am: Adapt.
* module/web/uri.scm (uri-error): New proc, throws to 'uri-error.
(validate-uri, uri-decode, uri-encode): Use uri-error.
* test-suite/tests/web-uri.test: Update for uri-error.