Consider making it easier to use without flakes and overlays

[CobaltCause]

Assuming rust-overlay is bound to a path to the rust-overlay source, and nixpkgs is bound to an imported nixpkgs instance, currently you have to do this:

nixpkgs.lib.fixedPoints.fix (rust-bin:
  import "${rust-overlay}/lib/rust-bin.nix" {
    inherit (nixpkgs) lib pkgs;
    inherit (rust-bin) nightly;
    manifests = import "${rust-overlay}/lib/manifests.nix" {
      inherit (nixpkgs) lib;
      distRoot = import "${rust-overlay}/lib/dist-root.nix";
    };
  }
)

which is a mouthful, and I suspect these file paths and such are not considered public API. It would be nice to be able to use this more easily without flakes and overlays.