st.c: skip all deleted entries [Bug #17779]

Update the start entry skipping all already deleted entries.
Fixes performance issue of `Hash#first` in a certain case.

Author: tompng

benchmark/hash_first.yml

@@ -0,0 +1,11 @@
+prelude: |
+  hash1 = 1_000_000.times.to_h { [rand, true]}
+  hash2 = hash1.dup
+  hash2.keys[1..100_000].each { hash2.delete _1 }
+  hash2.delete hash2.first[0]
+
+benchmark:
+  hash1: hash1.first
+  hash2: hash2.first
+
+loop_count: 100_000

st.c

@@ -1244,8 +1244,13 @@ update_range_for_deleted(st_table *tab, st_index_t n)
 {
     /* Do not update entries_bound here.  Otherwise, we can fill all
        bins by deleted entry value before rebuilding the table.  */
-    if (tab->entries_start == n)
-        tab->entries_start = n + 1;
+    if (tab->entries_start == n) {
+        st_index_t start = n + 1;
+        st_index_t bound = tab->entries_bound;
+        st_table_entry *entries = tab->entries;
+        while (start < bound && DELETED_ENTRY_P(&entries[start])) start++;
+        tab->entries_start = start;
+    }
 }
 
 /* Delete entry with KEY from table TAB, set up *VALUE (unless