Based on Konstantin Ritt work posted on mailing list

Fixes https://github.com/harfbuzz/harfbuzz/pull/1432

Part of https://github.com/harfbuzz/harfbuzz/issues/1682

This reverts commit d219f899.

API change: Un-deprecate font kern API.

Part of https://github.com/harfbuzz/harfbuzz/issues/1682

We should document that this API is only necessary to hook up
non-OpenType kerning.  hb-ot-font will continue to NOT implement them.

Can be useful when using HarfBuzz for font loading and shaping
but using DirectWrite for rendering.

It is rather confusing to have script, language etc, in hb-common section
while feature is in hb-shape section. I keep looking for it in hb-common
section then using the API index because I can’t find it there.

New API:
+hb_aat_layout_has_substitution()
+hb_aat_layout_has_positioning()

Fixes https://github.com/harfbuzz/harfbuzz/pull/1346

New version has axis flags.

New API:
+hb_ot_var_axis_info_t
+hb_ot_var_find_axis_info()
+hb_ot_var_get_axis_infos()

Deprecated API:
-HB_OT_VAR_NO_AXIS_INDEX
-hb_ot_var_axis_t
-hb_ot_var_find_axis()
-hb_ot_var_get_axes()

Fixes https://github.com/harfbuzz/harfbuzz/issues/1241

Part of https://github.com/harfbuzz/harfbuzz/issues/1241

New API:
+hb_ot_var_axis_flags_t
+hb_ot_var_axis_get_flags

https://github.com/harfbuzz/harfbuzz/pull/1254

https://github.com/harfbuzz/harfbuzz/commit/d941f66c75fe26f909b1ba248535cc372bbde851#commitcomment-31076011

This is like hb_script_t.

We had this exposed as unsigned int since 2.0.0 release in two APIs,
as well as hb_ot_layout_get_size_params() from earlier.
But since no one uses those (right?!), let's just fix this now.

And swap return value positions, so can be annotated for gobject-introspectin.

* hb_ot_layout_feature_get_characters
* hb_ot_layout_feature_get_name_ids

However HarfBuzz currently doesn't expose an API for retrieving the actual
information associated with NameId from the `name` table and that should be
done separately.

`hb_ot_tags` replaces `hb_ot_tags_from_script` and
`hb_ot_tag_from_language`.

`hb_ot_layout_table_select_script` replaces
`hb_ot_layout_table_choose_script`.

`hb_ot_layout_script_select_language` replaces
`hb_ot_layout_script_find_language`.