* Add length param for seralization write_string

to increase a little bit the performance for big strings.
* Change the format of storing strings for serial-bin.

doc/API.txt

@@ -50,4 +50,4 @@ V0.4.*: * The order of a list initialized using M_LET is reversed
         to match what is done for ARRAY (and the C++ forward_list).
         * Rename M_SKIP, M_KEEP & M_MID into M_SKIP_ARGS, M_KEEP_ARGS & M_MID_ARGS
         due to conflict with glibc malloc.h (that defines M_KEEP for compatibility).
-        
+        * Method write_string of m_serial expects a new argument (number of characters)