struct FieldArenaRep

This struct maps field types to the types that we will use to represent them

when allocated on an arena. This is necessary because fields no longer own an

arena pointer, but can be allocated directly on an arena. In this case, we

will use a wrapper class that holds both the arena pointer and the field, and

points the field to the arena pointer.

Additionally, split pointer fields will use this representation when

allocated, regardless of whether they are on an arena or not.

For example:

```

template

<

>

struct FieldArenaRep

<Message

> {

using Type = ArenaMessage;

static Message* Get(ArenaMessage* arena_rep) {

return

&arena

_rep->message();

}

};

```