Use segmented list to implement `Hash` [Experimental]

I know it's not hash at all, but reduce memory consumption.

include/mruby/hash.h

@@ -18,7 +18,7 @@ MRB_BEGIN_DECL
 struct RHash {
   MRB_OBJECT_HEADER;
   struct iv_tbl *iv;
-  struct kh_ht *ht;
+  struct seglist *ht;
 };
 
 #define mrb_hash_ptr(v)    ((struct RHash*)(mrb_ptr(v)))
@@ -76,7 +76,7 @@ MRB_API mrb_value mrb_hash_get(mrb_state *mrb, mrb_value hash, mrb_value key);
  *
  * Equivalent to:
  *
- *     hash.hash_key?(key) ? hash[key] : def
+ *     hash.key?(key) ? hash[key] : def
  *
  * @param mrb The mruby state reference.
  * @param hash The target hash.
@@ -199,7 +199,6 @@ KHASH_DECLARE(ht, mrb_value, mrb_hash_value, TRUE)
 #define RHASH_TBL(h)          (RHASH(h)->ht)
 #define RHASH_IFNONE(h)       mrb_iv_get(mrb, (h), mrb_intern_lit(mrb, "ifnone"))
 #define RHASH_PROCDEFAULT(h)  RHASH_IFNONE(h)
-MRB_API struct kh_ht * mrb_hash_tbl(mrb_state *mrb, mrb_value hash);
 
 #define MRB_HASH_DEFAULT      1
 #define MRB_HASH_PROC_DEFAULT 2