Deprecate components of output lists

[krlmlr]

Examples:

• parent vs. father in chore: use "parent" instead of "father" in bfs() and dfs() #1523
• biconnected_components() returns invalid result fields #1203
• ...

We could put an ALTREP object that gives a warning when accessed. Only useful in the very long term.

[maelle]

Is there an example of a package where this is implemented?

[krlmlr]

I'm no longer sure ALTREP is a good solution here.

For affected functions, we can add an argument format = "v2", with "v1" being an option that will be deprecated and defunct at some point.

[krlmlr]

Or "editions" as in testthat?

[krlmlr]

We could declare the edition when constructing the igraph object. Deserialized igraph objects would default to 1, newly constructed would default to 2.

[maelle]

But what should be the default of the function, the new or old version? The new one I presume, which means it's still a breaking change?