Co-Authored-By: Nn8willis <nwillis@glyphography.com>

Co-Authored-By: Nn8willis <nwillis@glyphography.com>

Co-Authored-By: Nn8willis <nwillis@glyphography.com>

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11210

Speeds up runnings in-house tests from over 20s to 2s.

I keep forgetting that primitive types are NOT initialized during construction. :|

See a85641446c30247c4e948263f0f8c1147ed4efb9

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.

It was arbitrary that this struct had it and not dozens of others.