add FS-1331 - Allow opens in type and expression scopes

[ijklam]

Click “Files changed” → “⋯” → “View file” for the rendered RFC.

[Tarmil]

It looks like open type hasn't been considered. I think people will expect it to work, consistently with module-level open type, so the RFC should either include it, or explain why it doesn't.

Edit: actually it is present in the examples. Still, I think a mention in the text would be welcome.

[Martin521]

Can a section "Changes to the F# spec" be added?

[T-Gro]

That makes sense. I will re-review once spec changes are added.