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

New API:
+hb_ot_var_axis_flags_t
+hb_ot_var_axis_get_flags

We're not going to define public macros for tags for tables.
There's little use to those and they are implementation detail.

The 'avar' table does not allow random access to axis maps,
so change API to avoid quadratic-time implementation.

Removed -hb_ot_var_normalize_axis_value(), added
+hb_ot_var_normalize_variations() and
+hb_ot_var_normalize_coords() instead.

Supports enumerating variation axes, normalizing values, etc.

This prepares the headers for exporting symbols using visibility
attributes or __declspec(dllexport), so that we do not need to maintain
symbols listing files, as this is what was and is done in GLib and GTK+.

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

This simplifies the code, reduces static data, and makes the design
more extensible to future additions of new scripts.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.