Using decltype() instead.

[skip ci]

[skip ci]

[skip ci]

c:\projects\harfbuzz\src\hb-ot-color-cbdt-table.hh(59): warning C4146: unary minus operator applied to unsigned type, result still unsigned [C:\projects\harfbuzz\build\harfbuzz.vcxproj]

"Test" as in compiles..

Seprate namespace, cleaner, more clear.

See 6b6783e1

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.

I think Apple does very similarly, but probably with a stack size of 16.
We do it with a stack size that is currently set to 64.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1531