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

shell.nix

@@ -0,0 +1,34 @@
+{
+  npins ? import ./npins,
+  nixpkgs ? npins.nixpkgs,
+  pkgs ? import nixpkgs { },
+  lib ? pkgs.lib,
+  mkShell ? pkgs.mkShell,
+  sqlite ? pkgs.sqlite,
+  openssl ? pkgs.openssl,
+  rust-analyzer ? pkgs.rust-analyzer,
+  rustc ? pkgs.rustc,
+  cargo ? pkgs.cargo,
+  pkg-config ? pkgs.pkg-config,
+  crate2nix ? pkgs.crate2nix,
+  protobuf ? pkgs.protobuf,
+  datasette ? pkgs.datasette,
+  ...
+}:
+mkShell {
+  name = "sidx-shell";
+  nativeBuildInputs = [
+    (lib.getBin sqlite)
+    cargo
+    crate2nix
+    rustc
+    rust-analyzer
+    pkg-config
+    protobuf
+    datasette
+  ];
+  buildInputs = [
+    openssl
+    sqlite
+  ];
+}