Rust graph

[seddonym]

Got it working, but it's very slow!

Things to try:

• Replacing Module{ String } with Module { Arc<String> }.
• String interning for module names.
• Preallocating a size for certain hashmaps.
• Rather than repeatedly cloning the whole Graph struct, could we just clone the inner imports StableGraph?
• Could we avoid mutating the graph altogether when checking for imports? We brought in module squashing etc. to
  speed up the Python, but perhaps Rust is fast enough that we don't need to bother.

[codspeed-hq]

CodSpeed Performance Report

Merging #166 will degrade performances by 99.94%

_{Comparing rust-graph (43235d9) with master (6523dfe)}

Summary

⚡ 4 improvements
❌ 5 regressions
✅ 2 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	rust-graph	Change
❌	test_chain_found	67.1 µs	65,091.4 µs	-99.9%
❌	test_no_chain	39.9 µs	64,891.2 µs	-99.94%
⚡	test_chains_found	128 ms	80.8 ms	+58.34%
⚡	test_no_chains	128.7 ms	80.4 ms	+60.08%
❌	test_build_django_from_cache	224.9 ms	442.5 ms	-49.17%
❌	test_deep_layers_large_graph	1.5 s	5.9 s	-74%
⚡	test_find_downstream_modules	2,489.4 ms	9 ms	×280
⚡	test_find_upstream_modules	1,828.1 ms	3.3 ms	×550
❌	test_top_level_large_graph	784.1 ms	4,597.5 ms	-82.94%