DIP Proposal: @manualScoped to prevent automatic field destruction

[FeepingCreature]

Update:

https://github.com/dlang/dmd/pull/5830

Turns out unions with fields with destructors is **define 
behavior**. I did not know this. Probably the spec needs updating.

So that pretty much alleviates the need for this DIP. Though I 
kind of half-believe that this DIP is actually *better* than 
unrestricted unions, so I'd still leave it open.