Merge declaration and init of variable in ws_string_raw()

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Fix: ws_string_set_from_raw() should return int

This patch changes ws_string_set_from_raw() to return int. Zero is
returned on success, a negative errno number on failure.

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Adapt ws_string_set_from_raw() calls to check return value (in deserializer callback code)

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Add likely/unlikely branch optimization macros in string.c

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Re-setting of self->str is not required here

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
We can simply return true here

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Remove (void) cast for u_str{From,To}UTF8() return value

Pair-programmed-with: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>