Make IOptionalValueInternals public

[EvilVir]

Or at least the IsSpecified flag should be public. It would greatly help in some scenarios like Linq filtering etc.

[desjoerd]

I try to keep the surface area as limited as possible as I want to treat it as a .NET Library and breaking changes are no option. By limiting the api surface I limit the possibility of breaking changes.

I will think about it while looking at union types.