feat(library/unifier): add option 'unifier.unfold_opaque', remove option 'unifier.use_exceptions' (the user should not be able to change this)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/unifier): store the unifier_plugin in the environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel): move standard and hott kernel instantiations to library

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/unifier): move inductive datatype support to inductive_unifier_plugin

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>