[nothing to see here] lib: Add functionInfo

[infinisil]

Description of changes

Adds a powerful functionInfo function, giving you more information about your functions!

functionInfo builtins.add ->
  {
    type = "builtin";
  }

functionInfo (arg: arg + 1) ->
  {
    type = "argument";
    argumentName = "arg";
  }

functionInfo (arg@{ a }: a + 1) ->
  {
    type = "attributes";
    argumentName = "arg";
    attributes = { a = false; };
    ellipsis = false;
  }
functionInfo ({ a ? 0, ...}: a + 1) ->
  {
    type = "attributes";
    argumentName = null;
    attributes = { a = true; };
    ellipsis = true;
  }

PS: Don't look at the implementation

PPS: This PR is only half-serious, motivation came from #194514 (comment), where this PR would actually have a good use case (cc @roberth)

Things done

[roberth]

:O

I'll see myself out.

[roberth]

argumentName doesn't seem useful.

ellipsis : bool is slightly more useful, but I don't think it leads to an improvement. lazyFunction does have a "problem" with ..., which is that those don't end up in the @ param. However, we simply can't put anything in ... without figuring out the attribute names of the argument. Something like the lazy attribute names thing you've worked on might help, if mapAttrs (k: v: arg.${k}) getArgs // arg has the right behavior in such an implementation of lazy attribute names.

Interesting PR... :D