[stdlib] Make InlineFixedVector work on CollectionElement

[JoeLoser]

As mentioned at #3445 (comment), we can make InlineFixedVector work on memory-only types such as CollectionElement. In the latest nightly tonight, InlineArray and Span have been relaxed to work on CollectionElement instead of CollectionElementNew, which should unblock this work.

[gabrieldemarmiesse]

Isn't InlinedFixedVector deprecated? I know the behavior is slightly different from small buffer optimization, at least the one I implemented, but I'm not sure this behavior will be needed a lot once #2825 lands.

[JoeLoser]

Isn't InlinedFixedVector deprecated? I know the behavior is slightly different from small buffer optimization, at least the one I implemented, but I'm not sure this behavior will be needed a lot once #2825 lands.

Yeah, but it's unclear how long it will be before we can land SBO, that's all.

[gabrieldemarmiesse]

Fair enough!