(overlay_strings, recenter_overlay_lists): Fix typo in eassert in last commit.

(overlay_strings, recenter_overlay_lists): Fix typo in eassert in last commit.
(unchain_overlay): New function.
(add_overlay_mod_hooklist): Use AREF.
(copy_overlays, reset_buffer, overlays_at, overlays_in)
(overlay_touches_p, overlay_strings, recenter_overlay_lists)
(fix_overlays_in_range, fix_overlays_before, Fmake_overlay)
(Fmove_overlay, Fdelete_overlay, Foverlay_lists)
(report_overlay_modification, evaporate_overlays, init_buffer_once):
Adjust to new type of overlays_(before|after).