Change maktaba#rtp#Join to strip trailing slashes

Change maktaba#rtp#Join to strip trailing slashes, following the precedent of vim which doesn't include trailing slashes itself.

Paths are canonicalized again by maktaba#rtp#Split before being returned. Fixed minor bugs in Add and Remove where path comparisons would false-negative because paths weren't canonicalized.

Alternate fix for #72. Reverts the original workaround of always using forward slash regardless of 'shellslash' setting.
Bugfix here: This missing copy() meant that Join() would modify the list passed into it.