Before this commit, we were inferring whether an
inductive/structure/class were meta or not. This was bad since the user
had no clue whether the type was trusted (non meta) or not.
Moreover, users could get confused by this behavior and assume the
kernel was allowing trusted code to rely on untrusted one.
Now, eta and impredicativity are assumed to be always true.
Motivation: the rest of the system assumes eta.
Regarding impredicativity, we decided to support only the standard library.
