`lib.fileset`: Implicitly coerce lists to unions

[infinisil]

For all lib.fileset functions taking file sets as arguments, it would be possible to implicitly coerce list types to the union of their elements. So e.g.

let
  fs = lib.fileset;
in
fs.toSource {
  root = ./.;
  fileset = fs.difference
    (fs.unions [
      ./Makefile
      ./src
    ])
    (fs.unions [
      ./src/tests
      ./src/main.o
    ]);
}

could be shortened to

let
  fs = lib.fileset;
in
fs.toSource {
  root = ./.;
  fileset = fs.difference
    [
      ./Makefile
      ./src
    ]
    [
      ./src/tests
      ./src/main.o
    ];
}

I did think of this while developing the library, but held back because I wasn't sure about the implications, especially regarding the duality of union and intersection.

But now that we decided against having a list version of intersection, it has become pretty clear that union is the one and only operation that would make sense to use by default for lists.

@alyssais would probably like this.

This issue is sponsored by Antithesis :sparkles:

[infinisil]

Oh and if we have this, we probably don't need lib.fileset.empty then, because [ ] is empty!