Add Builder::with for updating messages

Allow creating a new message based on an existing one, overriding
specific fields while copying the remaining original fields.

src/generator.rs

@@ -279,7 +279,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
         let tag = field_proto.number().unwrap();
         let f_type = field_proto.r#type().unwrap() as i32;
         let (rust_type, method) = map_type_to_rust_builder(f_type);
-        builder_fields.push((field_name, safe_name, tag, rust_type, method));
+        builder_fields.push((field_name, safe_name, tag as u32, rust_type, method));
     }
 
     // Builder struct — one `_written: bool` flag per field
@@ -288,7 +288,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
     for (field_name, _, _, _, _) in &builder_fields {
         output.push_str(&format!("    {}_written: bool,\n", field_name));
     }
-    output.push_str(&format!("}}\.\n\nimpl<'b> {}Builder<'b> {{\n", msg_name));
+    output.push_str(&format!("}}\n\nimpl<'b> {}Builder<'b> {{\n", msg_name));
 
     // Constructor — initialise every flag to false
     output.push_str(&format!(