Copy data into a stralloc
#include <stralloc.h>
int stralloc_copys(stralloc* sa,const char* buf);
stralloc_copys copies a \0-terminated string from buf into sa, without the \0. It is the same as stralloc_copyb(&sa, buf, str_len(buf)).
The data that sa previously contained is overwritten and truncated.
If stralloc_copys runs out of memory, stralloc_copys leaves sa alone and return 0, otherwise it returns 1.
1 for success, 0 on memory allocation failure.