[docs] LibraryEvolution: Weaken "fixed-layout" structs to "fixed-contents".
[docs] LibraryEvolution: Weaken "fixed-layout" structs to "fixed-contents".

This drops the requirement that a fixed-layout struct only contain fixed-size
things (which wasn't specified correctly anyway). Slava and Dmitri both
pointed out that the previous definition had several flaws, including not
really scaling to generics. This reduced version only promises that stored
instance properties won't be added, removed, or modified, which is still
enough to do useful optimizations. And in most cases, we'll be able to
infer the full "fixed-size" property of a type from this attribute anyway.

The name "fixed-contents" was chosen to force us to pick a better name later.

The next commit will bring back the "fixed-size" property as a performance
assertion.

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