feat(util/sexpr/options): serialization for options

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(util/sexpr): remove unnecessary decls

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/level): serializer for level objects

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/expr): serializer for kernel expressions

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