Implement Clone and Debug for SccGraph

[NotLeonian]

Implement the Clone and Debug traits for SccGraph.
Note that this requires implementing Clone and Debug for internal_scc::SccGraph #147 .

The Clone trait can be derived.
For the Debug trait, discussion is needed regarding the output format.
I think that either of the following approaches would be appropriate:

1. Derive it.
2. Since deriving would result in redundant output like SccGraph { internal: SccGraph {, format it the same way as the fmt function in internal_scc::SccGraph.

[NotLeonian]

Do you have any opinions on which of the two output formats I proposed is preferable, or whether another format would be better?
I personally think the latter one I proposed is the better option.

[NotLeonian]

I reconsidered and realized that arbitrarily adopting a different output from derive is not ideal.
I think derive is the better choice.