Fix #6204: `refine` is exponential in the number of fresh evars that it creates.

It is actually polynomial with a big exponent, probably quartic. This was due
to the Proofview.unifiable algorithm that kept recomputing the free evars of
an evar info. We share the computation instead.

This does not make the contrived example compile in a reasonable amount of time,
but it does make smaller instances compile way quicker than before. Indeed, the
example is essentially quadratic in size as all evars refer to the previously
defined ones in their signature.

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