Repeated primitive fields do not default to packed output in proto3.

[rcorre]

Given the following proto file:

syntax = "proto3";

package api;

message Example {
    repeated uint32 i = 1; 
}

And the following rust code:

let mut example = api::Example::new();
example.i.extend_from_slice(&[0, 1, 2, 3, 4, 5]);
let bytes = example.write_to_bytes().unwrap();
std::fs::write("/tmp/out", bytes).unwrap();

Produces a 12-byte file:

$ ls -lh /tmp/out 
-rw------- 1 rcorre rcorre 12 Jan 15 16:46 /tmp/out
$ xxd /tmp/out 
00000000: 0800 0801 0802 0803 0804 0805            ............

If I change the field to repeated uint32 i = 1 [packed=false];, I get the same output.
If I change the field to repeated uint32 i = 1 [packed=true];, instead I get an 8-byte file:

$ ls -lh /tmp/out 
-rw------- 1 rcorre rcorre 8 Jan 15 16:50 /tmp/out
$ xxd /tmp/out 
00000000: 0a06 0001 0203 0405                      ........

For a proto3 file, packed should be the default:

In proto2 this is done using the field option [packed=true]. In proto3 it is the default.