Experimenting using unification when matching evar/meta free subterms
while before these were supposed to consider only syntactically.

Made the experiment to unify with all delta flags unset. Keeping the
same flags as for non evar/meta free subterms would lead to too much
successes, as e.g. "true && b" matching "b" when the
modulo_conv_on_closed_terms flag is set, which is the case for
rewrite. But maybe should we instead investigate to have the same
flags but with the restrict_conv_on_strict_subterms flag set. This
rules out examples like "true && b" unifying with "b" and this is
another option which is ok for compiling the stdlib without any
changes.
Adapting to current semantics of "simpl non-evaluable-cst"
A bit more information in debug tactic unification.