Make kernel reduction code parametric over the handling of universes,
allowing fast conversion to be used during unification while respecting the
semantics of unification w.r.t universes.
- Inside kernel, checked_conv is used mainly, it just does checking, while infer_conv
is used for module subtyping.
- Outside, infer_conv is wrapped in Reductionops to register the right constraints
in an evarmap.
- In univ, add a flag to universes to cache the fact that they are >= Set, the
most common constraints, resulting in an 4x speedup in some cases (e.g. HigmanS).