Split out petgraph into separate crates

[indietyp]

• visit deprecated, available in petgraph-core
• data deprecated, available in petgraph-core
• move IndexType to petgraph-core
• move EdgeType, and related to petgraph-core
• rework IndexType to make use of funty

[indietyp]

Progress Report

• petgraph-core: ✅
• petgraph-graph: ✅
• petgraph-graphmap: ✅
• petgraph-adjacency-matrix: ✅
• petgraph-csr: ❌
• petgraph-algorithms: ❌
• petgraph-matrix-graph: ❌
• petgraph-dot: ❌

Everything compiles with the default feature sets, and all tests pass.

• List has been renamed to AdjacencyMatrix, to better reflect the purpose
  quickcheck + serde: untested
• tests haven't been ported to their respective crates just yet
• cargo-hack: not run yet, needs quickcheck fix for negative cycle first
• everything has been converted to no-std, where possible, no-alloc.
• Setting everything to deprecated as default will un-deprecate one-by-one after Tracking Issue: Trait rework #552 is resolved.
• List and Csr no longer use Ix as NodeIndex, but (just like graph) use NodeIndex, because this lead to (imo) a violation of the "separation of concerns" principle as well as introduced other problems, as we cannot implement NodeRef or anything like that in the remote crates on Ix, we could implement a default impl in the core crate, but IMO that would be the wrong move (as we would once again muddy the waters of what is a node index, what is an edge index, etc)

[indietyp]

Progress Report

• petgraph-core: ✅
• petgraph-graph: ✅
• petgraph-graphmap: ✅
• petgraph-adjacency-matrix: ✅
• petgraph-csr: ✅
• petgraph-algorithms: ⌛
• petgraph-matrix-graph: ✅
• petgraph-dot: ❌

The algorithms crate is by far the most challenging one; some algorithms require specific graphs (like Graph or AdjacencyMatrix). Most likely, these imports can be removed, but this will take a bit more time. While rummaging around the insides, I took the liberty of moving traversal from visit to petgraph-algorithms; while doing so, I plan to rework them to modernize them a bit. (The old will stay in core like they are and are deprecated)

[indietyp]

Progress Report

• petgraph-core: ✅
• petgraph-graph: ✅
• petgraph-graphmap: ✅
• petgraph-adjacency-matrix: ✅
• petgraph-csr: ✅
• petgraph-algorithms: ✅
• petgraph-matrix-graph: ✅
• petgraph-dot: ❌

• All crates have been successfully ported, serde has been reworked.
• dot still lives in petgraph and is now backed by the dot crate, I am currently contemplating replacing it in favor of graphviz-rust, as it allows serialization and deserialization, in the future we might want to move it into petgraph-io
  (or something similar) and also implement other formats, like GEXF.
• sadly the dot feature now requires std as it uses std::io::Write :/
• The only thing now remaining are some compile errors I left behind, quickcheck, testing.
  • for proptest I was thinking to create a companion petgraph-proptest, which implements multiple strategies. Nothing short-term.
• I have decided against reworking the currently non-generic algorithms, as it would be redundant first converting them and then reworking the traits, they are currently gated behind a temporary feature gat.

[indietyp]

Progress Report

• New crates: petgraph-generators, petgraph-proptest

While trying to upgrade to proptest I noticed that our quicktest is really feasible in quicktest 1.0 and likely also doesn't correctly shrink. I have taken the liberty to transfer to proptest and completely reworked the shrinking and generation.

The code is now generic and can easily be implemented for all kinds of graphs as well is configurable, so if users do not like the default, they can just call default::graph_strategy from the implementation with their arguments and graph type. With this change, we're likely to also support proptest on all existing types!

Crates compile (again). Due to the very nature of what we're trying to do (separating the concerns, making things more maintainable, etc.), there are quite a handful of new crates; they are now grouped under crates/ to keep things clean (commit message of 795a994 mentions inspirations)

The only thing missing are now moving the tests (and testing^^)

[indietyp]

Progress Report

quite a lot of tests have been ported; remaining are:

• graph (directed)
• algorithms in directed
• list
• quickcheck (to proptest)
• stable-graph
• graphmap

[indietyp]

Phew, well, that took a bit longer than expected. Finally, all tests have been converted and rewritten. This whole thing not only separated everything into separate crates but also had the nice side-effect that tests are now more readable and in places where one expects them.

The only thing that needs to be added now is going through with cargo-hack and move the benches.

[indietyp]

Sadly things came in between me and being able to work on this, but after some major rework of all the benchmarks (which if I do say so, are a lot easier to read now), we're ready!

[indietyp]

This is quite a large PR 😅 (but it's mostly just moving stuff around). I'll wait for any comments until the end of the week (or until I get approval) and then merge them into the next branch.