Escape 'type' keyword in generated Rust code

Use raw identifiers for fields named "type" to avoid conflicts with the
Rust keyword. Fix field number indexing in tests.

src/lib.rs

@@ -520,7 +520,7 @@ mod tests {
 
         // Validate that fields appear in the expected relative order
         let field_numbers: Vec<u32> = acc.fields()
-            .map(|r| r.expect("Failed to decode field").0.field_number)
+            .map(|r| r.expect("Failed to decode field").1.field_number)
             .collect();
 
         let essential_fields = [1, 2, 3, 14, 16, 20];