This reverts commit 5daf3bd4.

If other atexit callbacks try to destruct the objects we destruct
in atexit callbacks, bad things will happen.

I'll come up with some other way to catch premature destruction
of HB-owned objects.

Fixes https://github.com/behdad/harfbuzz/issues/618

After 4a27c17e we do not need
the special GlyphID.cmp(hb_codepoint_t), so just make GlyphID
a typedef.

Fixes https://github.com/behdad/harfbuzz/issues/612

Patch from Jean Ghali on mailing list.

Fixes https://github.com/behdad/harfbuzz/issues/615

We'll see which bots I broke.

If offset is zero, we return Null() object.  Wasn't prepared for that.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4088

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4099

Apparently calibri.ttf does this:
https://github.com/fonttools/fonttools/pull/1094#discussion_r148933791

Fixes https://github.com/behdad/harfbuzz/pull/605

Part of fixing https://github.com/behdad/harfbuzz/pull/605

Found using `codespell -i 3 -w -I '../harf-whitelist.txt'`
whereas the whitelist contents were:
```
beng
iff
pres
ot
te
teh
```

Seems to work.  Yay!

Still to do: run kerning if GPOS doesn't have 'kern' feature.

Almost there.

Pushing this out early to catch bot errors since I'm using template tricks
we were not using in HarfBuzz before.

See https://github.com/behdad/harfbuzz/pull/592

Might help with https://github.com/behdad/harfbuzz/pull/592
Ie. bringing in qsort_r() prototype always.

https://travis-ci.org/behdad/harfbuzz/jobs/295039536#L3468
https://ci.appveyor.com/project/behdad/harfbuzz/build/1.0.697/job/dr1ujlntxafeqbej#L142

(And prevent it from happening in the future.)

This concludes https://github.com/behdad/harfbuzz/pull/568