[set] Add backwards iterator

New API:
- hb_set_previous()
- hb_set_previous_range()

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index 5df33a8..91faa0b 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -528,7 +528,9 @@
 hb_set_is_empty
 hb_set_is_equal
 hb_set_next
+hb_set_previous
 hb_set_next_range
+hb_set_previous_range
 hb_set_reference
 hb_set_set
 hb_set_set_user_data