[API] Add compose() and decompose() unicode funcs, rename other ones

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.
diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index 55b94a7..2b0ec99 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -49,6 +49,8 @@
   HB_UNICODE_FUNC_IMPLEMENT (general_category) \
   HB_UNICODE_FUNC_IMPLEMENT (mirroring) \
   HB_UNICODE_FUNC_IMPLEMENT (script) \
+  HB_UNICODE_FUNC_IMPLEMENT (compose) \
+  HB_UNICODE_FUNC_IMPLEMENT (decompose) \
   /* ^--- Add new callbacks here */
 
 /* Simple callbacks are those taking a hb_codepoint_t and returning a hb_codepoint_t */
@@ -67,13 +69,13 @@
 
   bool immutable;
 
-  /* Don't access these directly.  Call hb_unicode_get_*() instead. */
+  /* Don't access these directly.  Call hb_unicode_*() instead. */
 
   struct {
-#define HB_UNICODE_FUNC_IMPLEMENT(name) hb_unicode_get_##name##_func_t name;
+#define HB_UNICODE_FUNC_IMPLEMENT(name) hb_unicode_##name##_func_t name;
     HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS
 #undef HB_UNICODE_FUNC_IMPLEMENT
-  } get;
+  } func;
 
   struct {
 #define HB_UNICODE_FUNC_IMPLEMENT(name) void *name;