init

First attempt at both Rust and snix-castore.
I'll start by putting together a single-file CLI app,
no splitting modules out until needed, no separation of concerns.

Currently, just the fetching is implemented:

```
$ sidx pkgs/dev*/cuda-*/cuda/manifests/*.json
```

npins: init

default.nix

@@ -0,0 +1,51 @@
+{
+  npins ? import ./npins,
+  nixpkgs ? npins.nixpkgs,
+  pkgs ? import nixpkgs { },
+  lib ? pkgs.lib,
+  ...
+}:
+
+lib.makeScope pkgs.newScope (
+  self:
+  let
+    addProtobuf = oldAttrs: {
+      nativeBuildInputs = oldAttrs.nativeBuildInputs or [ ] ++ [
+        (self.callPackage ({ protobuf }: protobuf) { })
+      ];
+    };
+  in
+  {
+    sidx-crate2nix = (
+      self.callPackage ./Cargo.nix {
+        defaultCrateOverrides = pkgs.defaultCrateOverrides // {
+          sidx = oldAttrs: {
+            src =
+              let
+                fs = pkgs.lib.fileset;
+              in
+              fs.toSource {
+                root = ./.;
+                fileset = fs.unions [
+                  ./src
+                  ./Cargo.toml
+                  ./Cargo.lock
+                ];
+              };
+          };
+          snix-castore = addProtobuf;
+          # prost-wkt-types = addProtobuf;
+          # openssl-sys = oldAttrs: {
+          #   nativeBuildInputs = oldAttrs.buildInputs or [ ] ++ [
+          #     (self.callPackage ({ pkg-config }: pkg-config) { })
+          #   ];
+          #   buildInputs = oldAttrs.buildInputs or [ ] ++ [
+          #     (self.callPackage ({ openssl }: openssl) { })
+          #   ];
+          # };
+        };
+      }
+    );
+    sidx = self.sidx-crate2nix.rootCrate.build;
+  }
+)