String module

i'd've done this differently, setting newsize = p_str->size + p_str->size here; and reusing using it below when setting p_newstr->size;, and using sizeof(str_t)+(sizeof(char)*(newsize+1)) on the next line. i think it'd be clearer, since "size" is an overloaded term meaning both "length of the string" and "amount of memory allocated"