Make the SQLite struct opaque

With the API changes already done, notably .is_open(), the
implementation of the sqlite can and should now be private.

This also doesn't need to be #[repr(C)] anymore since the C API does
not need to access the fields in the struct.

src/dc_sqlite3.rs

@@ -12,9 +12,8 @@ use crate::x::*;
 const DC_OPEN_READONLY: usize = 0x01;
 
 /// A wrapper around the underlying Sqlite3 object.
-#[repr(C)]
 pub struct SQLite {
-    pub cobj: std::sync::RwLock<*mut sqlite3>,
+    cobj: std::sync::RwLock<*mut sqlite3>,
 }
 
 impl SQLite {