Add nullable{,Ref} construction functions

[wilzbach]

Such construction helpers are there for literally every other type in Phobos, even in the same module, e.g. refCounted, rebindable, tuple - so I assume it was just forgotten?

[dukc]

Everything seems correct to me. Good work.

[wilzbach]

Everything seems correct to me. Good work.

Thanks!
Btw I am trying to move a bit of stuff and ideas from phobos-next to mainline Phobos. If you (or someone else), has time - please see this issue and the filtered TODO wiki

[UplinkCoder]

please don't change existing unittest if there are no semantic reasons to do so.
unit-tests should be extended, not modified.

[wilzbach]

please don't change existing unittest if there are no semantic reasons to do so.
unit-tests should be extended, not modified.

Those are public unittest that are shown to the user in the documentation. Hence it does make sense to update the style there. Most users will just copy / paste the first example in the documentation.

[UplinkCoder]

Alright, this is a trivial and helpful change. No reason not to merge it

[UplinkCoder]

Auto-merge toggled on

[nordlow]

Thanks, @wilzbach!