[face] Introduce character-map API

New API:
+hb_face_collect_unicodes
+hb_face_collect_variation_selectors
+hb_face_collect_variation_unicodes
diff --git a/test/api/test-subset-codepoints.c b/test/api/test-subset-codepoints.c
index a2c40cd..affe931 100644
--- a/test/api/test-subset-codepoints.c
+++ b/test/api/test-subset-codepoints.c
@@ -33,7 +33,7 @@
   hb_face_t *face = hb_subset_test_open_font("fonts/Roboto-Regular.abc.format4.ttf");
   hb_set_t *codepoints = hb_set_create();
 
-  hb_subset_collect_unicodes (face, codepoints);
+  hb_face_collect_unicodes (face, codepoints);
 
   hb_codepoint_t cp = HB_SET_VALUE_INVALID;
   g_assert (hb_set_next (codepoints, &cp));
@@ -54,7 +54,7 @@
   hb_face_t *face = hb_subset_test_open_font("fonts/Roboto-Regular.abc.format12.ttf");
   hb_set_t *codepoints = hb_set_create();
 
-  hb_subset_collect_unicodes (face, codepoints);
+  hb_face_collect_unicodes (face, codepoints);
 
   hb_codepoint_t cp = HB_SET_VALUE_INVALID;
   g_assert (hb_set_next (codepoints, &cp));
@@ -75,7 +75,7 @@
   hb_face_t *face = hb_subset_test_open_font("fonts/Roboto-Regular.abc.ttf");
   hb_set_t *codepoints = hb_set_create();
 
-  hb_subset_collect_unicodes (face, codepoints);
+  hb_face_collect_unicodes (face, codepoints);
 
   hb_codepoint_t cp = HB_SET_VALUE_INVALID;
   g_assert (hb_set_next (codepoints, &cp));