lvalue iterators must declare __item_type__ as a reference.

Ugly, but does the job.

Still not satisfied with how I can enforce iterators only, but
seems to work for now.

Not C++11.

../../third_party/harfbuzz-ng/src/src/hb-map.hh(56,45):  warning: comparison of integers of different signs: 'const hb_codepoint_t' (aka 'const unsigned int') and 'hb_map_t::(anonymous enum at ../../third_party/harfbuzz-ng/src/src/hb-map.hh:169:3)' [-Wsign-compare]
    bool is_unused () const    { return key == INVALID; }

warning: '_WIN64' is not defined, evaluates to 0 [-Wundef]

Fixes #1548 hopefully

Fixes https://github.com/harfbuzz/harfbuzz/issues/1549

fixes issue #1547

Currently fails directwrite backend.

hb-directwrite.cc: In constructor 'TextAnalysis::TextAnalysis(const wchar_t*, uint32_t, const wchar_t*, DWRITE_READING_DIRECTION)':
hb-directwrite.cc:489:18: error: 'TextAnalysis::mText' will be initialized after [-Werror=reorder]
   const wchar_t* mText;
                  ^~~~~
hb-directwrite.cc:488:12: error:   'uint32_t TextAnalysis::mTextLength' [-Werror=reorder]
   uint32_t mTextLength;
            ^~~~~~~~~~~
hb-directwrite.cc:282:3: error:   when initialized here [-Werror=reorder]
   TextAnalysis (const wchar_t* text, uint32_t textLength,
   ^~~~~~~~~~~~
hb-directwrite.cc: In function 'hb_bool_t _hb_directwrite_shape_full(hb_shape_plan_t*, hb_font_t*, hb_buffer_t*, const hb_feature_t*, unsigned int, float)':