Towards fixing https://github.com/harfbuzz/harfbuzz/issues/1146

Fixes https://github.com/harfbuzz/harfbuzz/issues/1299

Removes anomaly I was seeing in cpal table trying to use implicit Null(NameID).

This speeds up Roboto shaping by ~10%.  I was hoping for more.
Still, good defense against lookups with many subtables.

Some clang versions define static_assert as a macro apparently, so we cannot
redefine it...

This reverts commit 94bfea0c.
This reverts commit 4e626278.

So we don't accidentally break it again.

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

Finally, unified!