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

Before we used to give up.  Now, just ignore error and continue processing.

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

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=924848

Fixes https://github.com/harfbuzz/harfbuzz/issues/1556

Adjust tests.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1532

Fixes MORX-31

Clang and gcc know not to warn in these cases, but not nonmainstream compilers

If compiler doesn't inline StructAtOffset, this was an error since we
only disable cast-align at call-site.  So, move the cast out.

../src/hb-machinery.hh: In instantiation of 'const Type& StructAtOffset(const void*, unsigned int) [with Type = unsigned int]':
../src/hb-font.cc:146:85:   required from here
../src/hb-machinery.hh:63:12: error: cast from 'const char*' to 'const unsigned int*' increases required alignment of target type [-Werror=cast-align]
 { return * reinterpret_cast<const Type*> ((const char *) P + offset); }
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/hb-machinery.hh: In instantiation of 'Type& StructAtOffset(void*, unsigned int) [with Type = unsigned int]':
../src/hb-font.cc:147:79:   required from here
../src/hb-machinery.hh:66:12: error: cast from 'char*' to 'unsigned int*' increases required alignment of target type [-Werror=cast-align]
 { return * reinterpret_cast<Type*> ((char *) P + offset); }
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We don't support that version anymore.