Rename dc_chat_t to Chat

This clears the way to start working on making the functions safe.
But small PRs are good PRs so let's get this rename out of the way and
have future PRs less noisy.

Also stop making this #[repr(C)] and start making fields that are not
used private. Lastly clean up some comments by moving them or
deleting them, so they make sense again after the translation.

src/dc_lot.rs

@@ -126,7 +126,7 @@ pub unsafe fn dc_lot_get_timestamp(lot: *const dc_lot_t) -> i64 {
 pub unsafe fn dc_lot_fill(
     mut lot: *mut dc_lot_t,
     msg: *const dc_msg_t,
-    chat: *const dc_chat_t,
+    chat: *const Chat,
     contact: *const dc_contact_t,
     context: &Context,
 ) {