object: dict: use b_strings as keys rather than plain c-strings.

now that strings can contain UTF-8 codepoints and null chars, the b_dict
api has been enhanced to accept keys as b_strings as well as regular c-strings.

keys are now stored as b_strings internally, to allow a wider range of
keys to be used.

object/dict.h

@@ -6,9 +6,11 @@
 #include <blue/core/btree.h>
 #include <blue/core/queue.h>
 
+struct b_string;
+
 struct b_dict_bucket_item {
 	b_queue_entry bi_entry;
-	char *bi_str;
+	struct b_string *bi_str;
 	struct b_object *bi_value;
 };