Extract minimum assembly pathways as directed acyclic graphs of fragments

[jdaymude]

Resolves #22, enabling minimum assembly pathway extraction. Subsumes #146 and #147.

Currently in draft, as there are a variety of final issues to work out:

• A pathway DAG's node labels are currently DOT-formatted string representations of their respective molecular fragments. This is not a format that Python packages like networkx or rdkit can draw as molecules.
• Double-check function-level documentation to ensure it reflects any interface changes.
• Write new integration tests for pathway functionality in both the Rust crate and Python package.
• Ensure performance-neutrality for benchmarks that have pathway reconstruction disabled.
• Write new benchmarks that measure the speed of pathway reconstruction.
• After benchmarking, consider filtering collected match removal orders down to the number of pathways requested at each assembly state instead of collecting them all and then filtering down only at the end.