Optimization - speedup of lookup of compound attributes

Remove unused _access_element abstraction, which was never overriden
but its lookup/invocation had quite significant cost.  Speedup of
simple method and property accesses varies between 10% and 20% (of
lgi-introduced overhead).
Optimization - cache attributes lookup

instead of always going through lookup of side category tables, cache
results directly in the type table (which means that lookup is done
automatically by Lua) or in _cached table (when element's category is
significant).  This optimization significantly cuts time needed for
lookup of type attributes.