Proposing std.typecons : Optional (with PR)

[FeepingCreature]

Since my rfc regarding deprecating `alias get this` in Nullable 
(see https://github.com/dlang/phobos/pull/7060 ) encountered no 
meaningful criticism (as I'm tactfully choosing to interpret the 
total absence of reaction), but somebody mentioned that a new 
type may have more success, since it doesn't have to justify 
itself in the same way as a change to existing behavior, I 
present `Optional`: https://github.com/dlang/phobos/pull/7065 .

To reiterate the difference:

`Nullable` is the standard type for "adding an undefined value 
state to an existing type". It's being abused as an `Optional` 
type. Why "abused"? Well:

1. `get` is **undefined** if Nullable is null

2. `get` is called implicitly.

This is not good behavior for an `Optional` type! Since my 
attempt to turn `Nullable` into an `Optional` type encountered 
zero enthusiasm and mild resistance, I've started a second 
parallel attempt to introduce an `Optional` type from scratch.

(Translation: I've copypasted `Nullable` and changed a few names.)

Opinion? Anyone? Anyone at all? I'm so alone...