Initial version of display/overlay strings is working.

Initial version of display/overlay strings is working.

 src/xdisp.c (set_iterator_to_next, get_visually_first_element): Use
 it->bidi_it.string.schars rather than it->string_nchars when
 testing whether we're beyond string end, because string_nchars is
 zero for strings that come from overlays and display properties.
 src/bidi.c (bidi_cache_iterator_state): Fix a bug with testing
 character positions against the cached range, when we use a
 stacked cache.
 src/dispextern.h (struct iterator_stack_entry): New member
 paragraph_embedding.
 src/xdisp.c (push_it, pop_it): Save and restore it.