Add Attachment struct, new iface for create-rel

The Attachment struct exists, but this makes it glaringly obvious that
I've made a bad interface. The create_release_attachment should only
accept one file at a time and the loop over all files should happen in
main.rs

I've changed the signature, removed the loops, and wired in the newer
error handling routines. Needs fixing at call sites.

src/structs/mod.rs

@@ -1,2 +1,15 @@
+use serde::{Deserialize, Serialize};
+
 pub mod release;
 pub mod repo;
+
+#[derive(Debug, Deserialize, Serialize)]
+pub struct Attachment {
+    id: usize,
+    name: String,
+    size: i64,
+    download_count: i64,
+    created: String, // TODO: Date-time struct
+    uuid: String,
+    download_url: String,
+}