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This provides a type-safe way for an action to express that it wants to be
called only once, or to capture move-only objects. It is a generalization of
the type system-evading hack in ByMove, with the improvement that it works for
_any_ action (including user-defined ones), and correctly expresses that the
action can only be used with WillOnce. I'll make existing actions benefit in a
future commit.

PiperOrigin-RevId: 440496139
Change-Id: I4145d191cca5655995ef41360bb126c123cb41d3