Improve support for shaping Egyptian Hieroglyphs

Improve support for shaping Egyptian Hieroglyphs

* src/composite.c (composition_gstring_lookup_cache): Renamed from
gstring_lookup_cache and made external.  All callers changed.
* src/composite.h (composition_gstring_lookup_cache): Add
prototype.
* src/font.c (Ffont_shape_gstring): Call
composition_gstring_lookup_cache and return the cached composition
if it is already in the cache.

* lisp/language/misc-lang.el (egyptian-shape-grouping): New
function.
(composition-function-table): Use egyptian-shape-grouping in
setting up compositions for Egyptian Hieroglyphs.  Fix the
composition setup for horizontal and vertical joiners.