benchmark single encode/decode that uses generics

[egonelbre]

Currently we generate separate funcs instead of having a single encode/decode per message, theoretically it could generate one:

func CodeUser[C picobuf.Coder](c C, u *User) {
   c.String(1, &u.First)
   c.String(2, &u.Last)
   c.Int(3, &u.Age)
   CodeAddress(c, 4, &u.Address)
}

However, there's a slight problem that this would require knowing CodeAddress at generation time, rather than being a method on the Address type itself. Nevertheless, this approach does look enticing. Especially, if you want to skip using proto files entirely.