Developer brewmanz: PrintStr<>: Allow assign from FlashStringHelper* and const char*; add some backwards/sideways compatibility

[brewmanz]

Add size_t write(const __FlashStringHelper bufFSH, size_t size) ... inline void clear() ... inline const char getString() ... const __FlashStringHelper * operator = (const __FlashStringHelper *str) ... const char * operator = (const char *str) ...