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Use std::function to implement type erasure in Action, wrapping the legacy
ActionInterface if necessary.

This makes functors / std::function the primary way to implement Action; the
existing ActionInterface implementations are handled through ActionAdaptor.
The existing actions are not (yet) migrated though; they'll pay the cost of one
additional indirection - but that should be negligible.

PiperOrigin-RevId: 226126137