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

src/q/latest-download.sql

@@ -0,0 +1,16 @@
+SELECT
+    s.id AS sample_id,
+    b.blake3,
+    s.epoch
+FROM
+    sidx_uri_sample AS s,
+    sidx_uri AS u,
+    sidx_blob AS b
+ON
+    s.uri_id = u.id
+    AND s.blob_id = b.id
+WHERE
+    u.uri = ?
+ORDER BY
+    s.epoch DESC
+LIMIT 1;