Constify and simplify some hash indexing functions.

* include/apr_hash.h
  (apr_hash_this_key, apr_hash_this_key_len, apr_hash_this_val): Constify
    the hash index parameter.

* tables/apr_hash.c
  (apr_hash_this_key, apr_hash_this_key_len, apr_hash_this_val): Simplify.
--This line, and those below, will be ignored--

Index: include/apr_hash.h
===================================================================
--- include/apr_hash.h	(revision 1432927)
+++ include/apr_hash.h	(working copy)
@@ -171,21 +171,21 @@ APR_DECLARE(void) apr_hash_this(apr_hash
  * @param hi The iteration state
  * @return The pointer to the key
  */
-APR_DECLARE(const void*) apr_hash_this_key(apr_hash_index_t *hi);
+APR_DECLARE(const void *) apr_hash_this_key(const apr_hash_index_t *hi);
 
 /**
  * Get the current entry's key length from the iteration state.
  * @param hi The iteration state
  * @return The key length
  */
-APR_DECLARE(apr_ssize_t) apr_hash_this_key_len(apr_hash_index_t *hi);
+APR_DECLARE(apr_ssize_t) apr_hash_this_key_len(const apr_hash_index_t *hi);
 
 /**
  * Get the current entry's value from the iteration state.
  * @param hi The iteration state
  * @return The pointer to the value
  */
-APR_DECLARE(void*) apr_hash_this_val(apr_hash_index_t *hi);
+APR_DECLARE(void *) apr_hash_this_val(const apr_hash_index_t *hi);
 
 /**
  * Get the number of key/value pairs in the hash table.
Index: tables/apr_hash.c
===================================================================
--- tables/apr_hash.c	(revision 1432927)
+++ tables/apr_hash.c	(working copy)
@@ -162,28 +162,19 @@ APR_DECLARE(void) apr_hash_this(apr_hash
     if (val)  *val  = (void *)hi->this->val;
 }
 
-APR_DECLARE(const void *) apr_hash_this_key(apr_hash_index_t *hi)
+APR_DECLARE(const void *) apr_hash_this_key(const apr_hash_index_t *hi)
 {
-    const void *key;
-
-    apr_hash_this(hi, &key, NULL, NULL);
-    return key;
+    return hi->this->key;
 }
 
-APR_DECLARE(apr_ssize_t) apr_hash_this_key_len(apr_hash_index_t *hi)
+APR_DECLARE(apr_ssize_t) apr_hash_this_key_len(const apr_hash_index_t *hi)
 {
-    apr_ssize_t klen;
-
-    apr_hash_this(hi, NULL, &klen, NULL);
-    return klen;
+    return hi->this->klen;
 }
 
-APR_DECLARE(void *) apr_hash_this_val(apr_hash_index_t *hi)
+APR_DECLARE(void *) apr_hash_this_val(const apr_hash_index_t *hi)
 {
-    void *val;
-
-    apr_hash_this(hi, NULL, NULL, &val);
-    return val;
+    return (void *)hi->this->val;
 }
 
 /*