swift234

[docs] Address some of Slava's feedback on LibraryEvolution.rst.

(but not all of it)

- Add a note about throwing functions becoming non-throwing.
- Clarify that let-properties may not be turned into read-only vars.
- Permit new static stored properties on fixed_layout structs.
- Drop note about "fixed_layout-since-first-available".
- Add note disparaging the utility of @fixed.
- Add note about adding defaulted associated types to protcols.
- Clarify which members can and cannot be added to a class.
- Add some class-specific permitted changes.
- Note that adding/removing 'dynamic' is /not/ permitted.
- Split up protocol and non-protocol extensions.
- Explain why the 'no_payload' performance assertion isn't needed.

Saving the rest of the feedback for the next major content addition.