When the new `ignore-empty-path-segments` feature is enabled, multiple leading slashes at the start of the path are ignored (the first segment starts after them), and multiple slashes at the end of any matched segment are ignored (so the next segment starts after them).  In addition, the `end` filter will ignore multiple slashes that follow the final segment. Both the `fullpath` and `tail` filters will still return any multiple slashes in their matched text, but their `segments` iterator will ignore the empty segments (in fact it does this today even without this feature being set).

This change does _not_ include this feature in the list of default features.

This change includes path filter testing on paths with multiple slashes in them.

This change fixes seanmonstar#738.

This fixes the new compiler warning about implicit associated type bounds, as per seanmonstar#1020 (comment)