RWARC should explicitly 'impl... Clone`

[jchinlee]

Presently RWARC implements a clone function, but the two words Clone for are missing, resulting in compiler error when calling clone.

[bblum]

It should derive clone, I think.

[jchinlee]

Even better!

[lilyball]

Deriving Clone means losing the docstring. Maybe not a big deal for RWArc, since it just references arc::clone, but the logical extension of this would be to derive Clone for Arc as well, and the docstring there is actually quite useful.

[bblum]

Oh, good point, thank you. Yes, they should probably keep the docstrings, even if just to point the reader at the docstring for Arc.