Loosen trait constraints and simplify structure for longest_path

[mtreinish]

In the recently merged #1192 a new generic DAG longest_path function was added to rustworkx-core. However, the trait bounds on the function were a bit tighter than they needed to be. The traits were forcing NodeId to be of a NodeIndex type and this wasn't really required. The only requirement that the NodeId type can be put on a hashmap and do a partial compare (that implements Hash, Eq, and PartialOrd). Also the IntoNeighborsDirected wasn't required because it's methods weren't ever used. This commit loosens the traits bounds to facilitate this. At the same time this also simplifies the code structure a bit to reduce the separation of the rust code structure in the rustworkx crate using longest_path().

[coveralls]

Pull Request Test Coverage Report for Build 9133077909

Details

• 33 of 33 (100.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 96.474%

---

Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	6	95.53%
<!--	Total:	6		-->

Totals
Change from base Build 9104623275:	-0.04%
Covered Lines:	16744
Relevant Lines:	17356

---

💛 - Coveralls