Nullable with auto-allocation on access

[Timothee Cour via Digitalmars-d]

Is there a construct similar to Nullable that would auto-allocate upon
access (set/get) if isNull is true?

Use case: https://github.com/msoucy/dproto/issues/117 [getters and
setters should work without calling `init` on Nullable fields, as in
C++ #117]

copied inline for easy reference:

```
in C++ we can write:

message MyMessage{
  optional Foo foo=1;
}
message Foo{
  optional Bar bar=1;
}
message Bar{
  optional string baz=1;
}

MyMessage a;
a.mutable_foo()->mutable_bar()->set_baz("hello");

in dproto we need to call init on the intermediate fields foo, bar

auto initialize_nullable(T:Nullable!U, U)(ref T a){ a=U.init; }

MyMessage a;
a.foo.initialize_nullable;
a.foo.bar.initialize_nullable;
a.foo.bar.baz="hello";

Would be nice to not have to call init and allow this:

MyMessage a;
a.foo.bar.baz="hello";

I believe this could be implemented via a modification of Nullable
that would call init on 1st access to a setter.
```