Fixes https://github.com/harfbuzz/harfbuzz/issues/1659

As was hit by the fuzzer.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=14032

Don't think we need hb_is_same().

MSVC seems to need it.

From Orvid King: TLDR; MSVC has some issues using sizeof(declval<T>()) for
SFINAE of templated types, so I just used SFINAE in a different context where
MSVC doesn't have the issue.

Tested, but feels fragile :(.

Assignment is invalid on invalid object.

Ugh.  Need to think of something else.

While doable with hb_deref_pointer() as well, we also would then
need to do it in a ton of places.  Not worth it / messy.

Also switch to better mixing.

No idea how to avoid dupicating code.  Was hoping hb_deref_pointer()
would do it, but looks like a pointer can't bind to a universal
reference T&&.  Humm.

Rarely used, but used, in name table or similar constructs.

Different initial and accumulator types

To be cleaned up.