Change semantics of wildcard patterns

This is a breaking API change that changes how wildcard patterns are
treated. In particular, wildcards are no longer allowed to appear at
arbitrary places in the URL, and are only allowed to appear immediately
after a path separator. This change effectively changes the wildcard
sigil from "*" to "/*".

Users who use wildcard routes like "/hello*" will have to switch to
regular expression based routes to preserve the old semantics.

The motivation for this change is that it allows the router to publish a
special "tail" key which represents the unmatched portion of the URL.
This is placed into URLParams under the key "*", and includes a leading
"/" to make it easier to write sub-routers.