improve subgraph count_nodes performance

[ljeub-pometry]

What changes were proposed in this pull request?

Refactor NodeSubgraph so it actually uses the NodeList optimisation

Why are the changes needed?

calling count_nodes on a subgraph was unnecessarily iterating through all nodes

Does this PR introduce any user-facing change? If yes is this documented?

No api changes

How was this patch tested?

the tests

[fabianmurariu]

LGTM

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rust Benchmark'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 2.

Benchmark suite	Current: cf63697	Previous: 0414b7f	Ratio
lotr_graph_subgraph_10pc/iterate edges	17999 ns/iter (± 93)	6570 ns/iter (± 104)	2.74
lotr_graph_subgraph_10pc_windowed/iterate edges	17421 ns/iter (± 70)	6678 ns/iter (± 20)	2.61
lotr_graph_subgraph_10pc_windowed/iterate_exploded_edges	21656 ns/iter (± 256)	10670 ns/iter (± 103)	2.03

This comment was automatically generated by workflow using github-action-benchmark.

[ljeub-pometry]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rust Benchmark'. Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 2.

Benchmark suite Current: cf63697 Previous: 0414b7f Ratio
lotr_graph_subgraph_10pc/iterate edges 17999 ns/iter (± 93) 6570 ns/iter (± 104) 2.74
lotr_graph_subgraph_10pc_windowed/iterate edges 17421 ns/iter (± 70) 6678 ns/iter (± 20) 2.61
lotr_graph_subgraph_10pc_windowed/iterate_exploded_edges 21656 ns/iter (± 256) 10670 ns/iter (± 103) 2.03
This comment was automatically generated by workflow using github-action-benchmark.

This is fixed