No other shaper will need shape_plan_data, by definition.  So, remove
abstraction layer and always create hb_ot_shape_plan_t as part of
hb_shape_plan_t.

This reverts commit 442a72d9.

Doesn't make sense.  No idea how my local compilers where happy with it!

Ouch :).

Although didn't need it...

Fix hb_blob_ptr_t::get_length () as well.

To remove custom null object next..

We never rely on that being equal to readonly.  Just not being
writable.  Maybe not even that given that the object is inert.

In prep for next commit, using default null pool.

Use in a couple of places.  Push to bots to see how many unhappy before
I convert the rest.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11312
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11305

Follow up on b4c61130

Fixes https://github.com/harfbuzz/harfbuzz/issues/1374

Fixes https://github.com/harfbuzz/harfbuzz/issues/1374

But fix UBSan complaint.

There's nothing in hb_bytes_t that guarantees lexical ordering, and
ordering by length first is much faster.

Ouch!

[colr/feat/trak] minor

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11327

Reported as https://github.com/noporpoise/sort_r/issues/7