Using decltype() instead.

Seprate namespace, cleaner, more clear.

I had used Null to make one of the bots happy before.  Not going
to bend to such demands anymore..

The mystery failure had to do with SFINAE failure because the template
function involved was accessing ::iter_t of a type that was also named iter_t.
In this context, apparently:

warning: ISO C++ specifies that qualified reference to 'iter_t' is a
constructor name rather than a type in this context, despite preceding 'typename' keyword
[-Winjected-class-name]

We use a new macro, also called hb_iter_t(), to get iterator type of
a type.  This uses declval/hb_decltype, and has the added benefit
that it returns correct type for const vs non-const objects, if they
have different iterators.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1553

Untested.

Untested.

It was only used for C++<11 which does not allow default parameters
in function templates.  Looks like we cannot support <11 anyway, so,
start cleaning up.

The right condition to check for would have been "is_struct", which
we don't have.

Fix ambiguity of hb_sorted_array_t::item_t with gcc.  No idea if that's a gcc bug
or what spec requires, but using aliasing using seems to fix it.  It probably breaks
our non-C++11 bots, in which case I have to condition the change.  Testing.