Rework Topological sort

[aesteve]

Topological Sort, reworked

Description

I was reading through Kahn's algorithm and topological sort and got a bit confused by the existing implementation, notably by the visited set which is declared, written to, but never read from.

So I went ahead and rewrote it the following way:

• use a Vec of pairs to describe the graph, rather than a BTreeMap
• relax the constraint for Ord for Nodes, and use Hash+Eq instead, using a HashMap instead of a BTreeMap (it sounds a bit more usual to me, but that's definitely opened to discussions)
• get rid of the visited set which wasn't used
• add a check at the end of the algorithm to check if we have visited every node,
  • if so, return the topologically sorted nodes
  • if not, return an Error (=> means the algorithm now detects cycles, and doesn't only partially sort)
• I've added some tests, one coming from the Wikipedia page on topological sort, the other being a cyclic graph

Definitely open to discussions about the implementation and/or changes I've made. No problem, I mostly did this for learning purposes.

Type of change

Please delete options that are not relevant.

• Breaking change since I changed the API a bit.

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

[siriak]

Looks good, thanks!