Part of https://github.com/harfbuzz/harfbuzz/issues/1648

To fix bogus MSVC warnings:

  c:\projects\harfbuzz\src\hb-array.hh(189): warning C4521: 'hb_array_t<Type>': multiple copy constructors specified [C:\projects\harfbuzz\build\harfbuzz.vcxproj]
  c:\projects\harfbuzz\src\hb-array.hh(189): warning C4522: 'hb_array_t<Type>': multiple assignment operators specified [C:\projects\harfbuzz\build\harfbuzz.vcxproj]

Exposed by:

$ make CPPFLAGS=-O0

Looks like static methods that do not get inlined end up exported.
We have a lot more.  Need to protect all at some point.  Wish there
was an easier way, like the visibility flag we pass that automatically
hides all inline methods.

Was exposed by check-symbols.sh when compiling on OS X 10.14 with:

$ make CPPFLAGS=-Oz CXXFLAGS=-flto=thin LDFLAGS=-lc++

MSVC seems to need it.

While doable with hb_deref_pointer() as well, we also would then
need to do it in a ton of places.  Not worth it / messy.

Also switch to better mixing.

Should be improved for hb_bytes_t.

Not enforcing it using type hierarchy.

We cannot rely on constexpr functions...

lvalue iterators must declare __item_type__ as a reference.

Ugly, but does the job.

hb_array_t is its own iterator...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1502

I think I like to keep this hb_supplier_t thing separately from hb_array_t.