Rethink how custom default values are treated.

[ergl]

Right now we're treating custom default values (proto2) as default assignments to the fields:

// proto
message Message
    optional int32 foo = 1 [default = 10];
}

// pony
class Message
  var foo: (I32 | None) = 10

For oneof fields, if several options have a custom default value:

// proto
message Message {
    oneof test {
        int32 foo = 1 [default = 10];
        int32 bar = 2 [default = 20];
    }
}

// pony
class Message
  var test: ((MessageFooField, I32) | (MessageBarField, I32) | None) = // ???

we have to choose which default value to use, but ideally we'd support all. Right now we're doing LWW, meaning, last default value (in field number order) wins.