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.

Sorry for the noise, downstream custom builders.  Please adjust.

Fixes #791.

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

We'll see which bots I broke.

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

Shape-plan caching is not implemented.

This one:

  map->mask = (1 << (next_bit + bits_needed)) - (1 << next_bit);

before the fix, the shift was done as an int, causing overflow
if it ever got to 1 << 31.  Sprinkle 'u's around.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=634805

Addressing Nikolay Sivov reviews on harfbuzz mailing list

No longer noticeable memory increase on create/destroy iterations,
highly better than current state of uniscribe backend

This reduces memory consumption of my iterated font create/destroy cycle test
significantly and makes it much better than uniscribe backend even

With help of
https://dxr.mozilla.org/mozilla-central/source/gfx/2d/NativeFontResourceDWrite.cpp