[set] Add backwards iterator

New API:
- hb_set_previous()
- hb_set_previous_range()

src/hb-set.cc

diff --git a/src/hb-set.cc b/src/hb-set.cc
index f3b864c..07cf9d0 100644
--- a/src/hb-set.cc
+++ b/src/hb-set.cc
@@ -437,7 +437,9 @@
  * @set: a set.
  * @codepoint: (inout):
  *
- * 
+ * Gets the next number in @set that is greater than current value of @codepoint.
+ *
+ * Set @codepoint to %HB_SET_VALUE_INVALID to get started.
  *
  * Return value: whether there was a next value.
  *
@@ -451,6 +453,26 @@
 }
 
 /**
+ * hb_set_previous:
+ * @set: a set.
+ * @codepoint: (inout):
+ *
+ * Gets the previous number in @set that is slower than current value of @codepoint.
+ *
+ * Set @codepoint to %HB_SET_VALUE_INVALID to get started.
+ *
+ * Return value: whether there was a previous value.
+ *
+ * Since: 1.8.0
+ **/
+hb_bool_t
+hb_set_previous (const hb_set_t *set,
+		 hb_codepoint_t *codepoint)
+{
+  return set->previous (codepoint);
+}
+
+/**
  * hb_set_next_range:
  * @set: a set.
  * @first: (out): output first codepoint in the range.
@@ -459,6 +481,8 @@
  * Gets the next consecutive range of numbers in @set that
  * are greater than current value of @last.
  *
+ * Set @last to %HB_SET_VALUE_INVALID to get started.
+ *
  * Return value: whether there was a next range.
  *
  * Since: 0.9.7
@@ -470,3 +494,26 @@
 {
   return set->next_range (first, last);
 }
+
+/**
+ * hb_set_previous_range:
+ * @set: a set.
+ * @first: (inout): input current first and output first codepoint in the range.
+ * @last: (out): output last codepoint in the range.
+ *
+ * Gets the previous consecutive range of numbers in @set that
+ * are greater than current value of @last.
+ *
+ * Set @first to %HB_SET_VALUE_INVALID to get started.
+ *
+ * Return value: whether there was a previous range.
+ *
+ * Since: 1.8.0
+ **/
+hb_bool_t
+hb_set_previous_range (const hb_set_t *set,
+		       hb_codepoint_t *first,
+		       hb_codepoint_t *last)
+{
+  return set->previous_range (first, last);
+}