Auto merge of #21788 - emilio:smallvec-union-not-quite-there-yet, r=asajeffrey

Partially revert #21746.

This reverts the relevant bits from #21746 so that style and dependencies can
build with stable.

This is important because:

 * `selectors` is a published crate.

 * Gecko compiles with stable (more or less).

I reviewed that PR under the assumption that the union feature was stable, since
untagged unions are stable since 1.19, but turns out that smallvec uses non-Copy
types in unions, which are still unstable.

This leaves the union feature used on Servo, so that it gets testing, taking
advantage of features being additive.

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