Assumes graphemes only have one base glyph.

...like UnsizedArrayOf<>.

This fixes a class of crasher bugs, mostly with color and AAT tables.  We
cannot use nullable offsets to varsized data that does not declare min_size,
because it's nost safe to use our fixed-size null pool for types that have
their size external.  So, use non_null'able offsets for these.

A further enhancement would be to make use of min_size in Null<> itself.
Will try that after.

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

In collaboration with Behdad

Splitted from #950 and #986 IIRC Chromium had a policy about this encouraging it,
not sure about automated way to detect and add them but for now lets have the needed
ones of them.

We always just used "unsigned int" for counter values. There's
no use for uint16_t outside of a struct.

Also, no need for explict casting where implicit does.

Maybe we should make DEFINE_* make structs uncopyable.