Migrate to a Cargo workspace, to keep library and binary distinct

[Xophmeister]

Is your feature request related to a problem? Please describe.
Many of the dependencies and functionality are relevant only to the Topiary library or binary, but rarely both (besides transitive dependency). It will make the code more maintainable, as additional functionality and adjacent tools are developed, to demarcate a clear separation.

Describe the solution you'd like
In #294, I have restructured the binary's source files, such that they have their own directory. This allows for binary-only modules to sit alongside it, rather than polluting the library namespace. This is a halfway-house to using Cargo workspaces, which will additionally allow the separation of dependencies (e.g., the library doesn't care about tempfile, etc.) and the isolation of any future tools (e.g., debuggers, visualisers, playground, etc.).

Describe alternatives you've considered
n/a

Additional context
The Cargo workspace should, in principle, be straightforward to configure. It wasn't added as part of #294 because it requires additional work to be compatible with the Nix setup.

A workspace may also be prerequisites (or, at least, highly desirable) for #137 and #171

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Tracking

• Separate library and binary code #303
• Convert Cargo project into a workspace #304
• Change Nix configuration to be faithful to the workspace layout #305