Disable copy/assign on them, as they shouldn't.

Make constructor / destructor call init_shallow/fini_shallow,
and make those idempotent.  So, these three can be constructed
on stack now and no init/fini call is needed.  As such,
hb_auto_t<> is not needed anymore.  I'll remove that separately.

Add pointer cast operator and plus operator.

From bungeman.

Fixes https://github.com/harfbuzz/harfbuzz/pull/1314

This reverts commit de0b9a46.
This reverts commit 921f0e6e.

Annnnd, revert.  MSVC doesn't like it.

Hopefully this fits fine with SFINAE with all our compilers.

Ugly, but...
Fixes https://github.com/harfbuzz/harfbuzz/issues/1227

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