Groups of slashes in path should be normalized and redirected

[davidism]

Originally reported at pallets/flask#1323.

/x and //x should route to the same location, possibly by redirecting. From the linked issue:

On most operating systems, extra slashes resolve to one. URLs came from file system notation so it would seem that the two should respond in the same way.

Apache removes duplicate slashes for a good reason.

Nginx has the merge_slashes directive, which is on by default.

RFC 3986 seems to imply you can't have multiple, contiguous slashes

Possible solution is to add a collapse_slashes option similar to strict_slashes.

Possibly related to #491.

[davidism]

This should apply to defining rules (//x becomes /x internally) as well as matching incoming paths (//x redirects to /x).