Fix and future-proof Path.clean separator removal (#1169)
Fix and future-proof Path.clean separator removal (#1169)

Fixed invalid negative indexing on a string, and made the deletion of
the trailing separator future-proof by using the .size() of the
separator instead of the fixed value of 1.
cquinn/ponyc,mkfifo/ponyc,dipinhora/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,jemc/ponyc,mkfifo/ponyc,Perelandric/ponyc,Theodus/ponyc,jupvfranco/ponyc,cquinn/ponyc,jupvfranco/ponyc,Theodus/ponyc,Theodus/ponyc,kulibali/ponyc,Theodus/ponyc,malthe/ponyc,boemmels/ponyc,sgebbie/ponyc,Praetonus/ponyc,sgebbie/ponyc,kulibali/ponyc,jupvfranco/ponyc,Theodus/ponyc,jupvfranco/ponyc,ponylang/ponyc,boemmels/ponyc,CausalityLtd/ponyc,jemc/ponyc,boemmels/ponyc,Perelandric/ponyc,cquinn/ponyc,kulibali/ponyc,sgebbie/ponyc,malthe/ponyc,ponylang/ponyc,Perelandric/ponyc,mkfifo/ponyc,jupvfranco/ponyc,mkfifo/ponyc,Perelandric/ponyc,sgebbie/ponyc,Praetonus/ponyc,kulibali/ponyc,Perelandric/ponyc,malthe/ponyc,Praetonus/ponyc,dipinhora/ponyc,cquinn/ponyc,jemc/ponyc,boemmels/ponyc,CausalityLtd/ponyc,Praetonus/ponyc,malthe/ponyc,mkfifo/ponyc,dipinhora/ponyc,boemmels/ponyc,ponylang/ponyc