add perllike hash function for strings

This also adds a new API json_global_set_string_hash() which permits
to select the hash function. The default one is the only one that was
previously present. So there are no changes to existing apps, and the
new hash function needs to be explicitely be opted in. Especially for
smaller strings, the perllike functions seems to be around twice as
fast as the other one, with similarly good results in value distribution.

linkhash.h

@@ -41,6 +41,23 @@ extern "C" {
  */
 #define LH_FREED (void*)-2
 
+/**
+ * default string hash function
+ */
+#define JSON_C_STR_HASH_DFLT 0
+
+/**
+ * perl-like string hash function
+ */
+#define JSON_C_STR_HASH_PERLLIKE 1
+
+/**
+ * This function sets the hash function to be used for strings.
+ * Must be one of the JSON_C_STR_HASH_* values.
+ * @returns 0 - ok, -1 if parameter was invalid
+ */
+int json_global_set_string_hash(const int h);
+
 struct lh_entry;
 
 /**
@@ -144,16 +161,6 @@ struct lh_table {
 };
 
 
-/**
- * Pre-defined hash and equality functions
- */
-extern unsigned long lh_ptr_hash(const void *k);
-extern int lh_ptr_equal(const void *k1, const void *k2);
-
-extern unsigned long lh_char_hash(const void *k);
-extern int lh_char_equal(const void *k1, const void *k2);
-
-
 /**
  * Convenience list iterator.
  */