Evar normalization is now done eagerly.

As witnessed in the ProjectiveGeometry contrib, some evar normalization were
taking ages because of an exponential behaviour due to a call-by-name
substitution: when normalizing an evar, its arguments were substituted right
away and the resulting term was then normalized, leading to a potential
duplication of normalizations.

Now we normalize evar arguments before substituting them, in a call-by-value
fashion. It makes examples from ProjectiveGeometry compile instanteanously
when they were killing the machine due to excessive memory allocation before
the patch.

Note that there is a tension here: we may be normalizing evar arguments too
eagerly, and try a call-by-need approach instead. To choose which particular
strategy we use, we should do some benchmarks... On stdlib, call-by-value
and call-by-need seem indistinguishable. To be continued?

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