[Version 13.0] Feature support for ref struct improvements

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal.

Notes on the MS proposal:

1. using statement: The Draft PR text in 13.14 The using statement|13.14.1 General, starting with “For ref struct resources, the only semantically equivalent formulation is” might need revision.
2. foreach statement: I made no changes. However, the MS spec text in 13.9.5.3, await foreach, starting with “- Otherwise, if E is a ref struct type and has an accessible Dispose() method, the finally clause is expanded to the semantic equivalent of:” might need revision.
3. Delegate type for the anonymous function or method group: I made no changes.
4. Inline arrays: I made no changes.
5. Soundness: I made no changes.
6. Open issues: I made no changes.
7. Considerations: I made no changes.