Fixes https://github.com/behdad/harfbuzz/issues/474

It is desirable to be able to build against older versions of glib.

fd7a245d changed the configure check to require glib > 2.38 for
G_TEST_DIST. Before that, version 2.16 was required, but in fact,
since aafe395a, G_PASTE is being used, which was introduced in 2.19.1.
And since 0ef179e2, hb-glib uses GBytes, which were introduced in
2.31.10.

2.19.1 is rather old, but 2.38 is rather new. For Firefox, building
against 2.22 is still supported, although we could probably get away
with bumping that to 2.28. Either way, GBytes is not available.

Arguably, if you build against a glib that doesn't support GBytes,
you're not going to use the hb_glib_blob_create function, so we hide
the function when building against such a glib.

As for G_TEST_DIST, when building against versions of glib that don't
support it, we can fallback to the previous behavior, which, AIUI, was
just making the test not work when building in a separate directory.

Based on data from http://upstream-tracker.org/versions/harfbuzz.html
Resolves #103

Rename HB_SCRIPT_CANADIAN_ABORIGINAL to HB_SCRIPT_CANADIAN_SYLLABICS
and a macro for the old name.

It's considerably faster than the fallback implementation we had
previously!

Based on patch from Philip Withnall.
https://bugs.freedesktop.org/show_bug.cgi?id=41095

Where causing issues for people with MSVC.

Add tests.

Adjust glib fallback implementation.

The tests are not hooked up for ICU yet.

Add compose() and decompose() unicode funcs.  These implement
pair-wise canonical composition/decomposition.

The glib/icu implementations are lacking for now.  We are adding
API for this to glib, but I cannot find any useful API in ICU.
May end of implementing these in-house.

Changed all unicode_funcs callback names to remove the "_get" part.
Eg, hb_unicode_get_script_func_t is now hb_unicode_script_func_t,
and hb_unicode_get_script() is hb_unicode_script() now.

Makes way for adding arbitrary user_data support.

So we can liberally use the simple features of C++ that parts of the
codebase is already using.

Also optimizes the common-direction script lookup to be an array lookup.

Unicode data providers can now be subclassed, including support for
chain-up.  The interface should now be nicely bindable, as well.

Also fix glib unicode funcs that where broken after hb_script_t
changes.  Nicely caught by the test-unicode.c added in this commit.