keyboard: use SmallVec for the Enter event's currently pressed keys

[valpackett]

The Event enum used string slices referencing stack-allocated Vecs (that were not reused anywhere else).
The lifetime makes it awkward for consumers (no way to e.g. store the Event somewhere) but I doubt
that it was a useful optimization (even if the optimizer can't allocate the Vec directly in the resulting enum,
creating a slice of it is hardly better than moving it: you still write the address and size, just not the capacity).

However, we can use SmallVec (already a dependency of wayland-rs) for an actual optimization:
this will avoid allocating Vec contents if 1-2 keys are held when the surface gets keyboard focus.

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Unfortunately this does change the public API (e.g. winit would need to drop a <'_>) — it's possible to add a PhantomData for the lifetime though, but accumulating backwards compat junk on 0.x versions is ehh :/