1) mk_meta now returns also the index of the created meta

1) mk_meta now returns also the index of the created meta
2) added datatypes for new patterns
3) added tactic nchange (using new patterns)
4) changed the type lowtac so that they no longer return the
   lists of open and closed goals, that are always computable
   in the new system
5) added some wrappers for kernel/refinement functions that
   work on statuses and terms labelled with their context
6) new implementation of select that performs subst-expansion.
   in-scope/out-scope used to mark regions selected by the pattern
7) ugly implementation of change based on re-opening of tagged
   subst entries