Add an invariant on future goals in Proof.run_tactic.

More precisely, we check that future goals retrieved in run_tactic
have no given_up goals since given_up goals are supposed to be
produced only by Proofview.given_up and put on the given_up store.

Doing the same for the shelf does not work: there is a situation where
run_tactic ends where the same goal is both in the comb and on the
shelf. This is when calling "clear x" on a goal "x:A |- ?p:B(?q[x])"
when the dependent goal "x:A |- ?q:C" is not on the shelf. Tactic
"clear" creates "|- ?p':B(?q'[])" and "|- ?q':C". The "advance" thing
sees that the new comb is now composed of ?p' and ?q' but ?q' is a
future goal which is later collected on the shelf (which ?q' is also
in the comb).

I tried to remove this redundancy but apparently it is
necessary. There is an example in HoTT (file
Classes/theory/rational.v) which requires this redundancy. I did not
investigate why: the dependent evar is created by ring as part of a
big term.

So, as a conclusion, I kept the redundancy.

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