Better encapsulation of future goals

We try to encapsulate the future goals abstraction in the evar map.
A few calls to `save_future_goals` and `restore_future_goals` are still
there, but we try to minimize them.

This is a preliminary refactoring to make the invariants between the
shelf and future goals more explicit, before giving unification access
to the shelf, which is needed for #7825.

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