ds: string: update to_string(); remove legacy b_stream callbacks

b_string will no longer implement the b_stream interface. writing to a string
via b_stream will now be handled by b_stringstream.

ds/include/blue/ds/string.h

@@ -81,7 +81,6 @@ static inline b_status b_string_tolower(b_string *str)
 {
 	return b_string_transform(str, tolower);
 }
-BLUE_API b_status b_string_open_stream(b_string *str, struct b_stream **out);
 
 BLUE_API b_status b_string_append_c(b_string *dest, char c);
 BLUE_API b_status b_string_append_wc(b_string *dest, b_wchar c);