Add generic function to grow an sds value

Move logic concerned with setting a bit in an sds to the SETBIT command
instead of keeping it in sds.c. The function to grow an sds can and will
be reused for a command to set a range within a string value.

src/sds.h

@@ -49,11 +49,11 @@ size_t sdslen(const sds s);
 sds sdsdup(const sds s);
 void sdsfree(sds s);
 size_t sdsavail(sds s);
+sds sdsgrowsafe(sds s, size_t len);
 sds sdscatlen(sds s, void *t, size_t len);
 sds sdscat(sds s, char *t);
 sds sdscpylen(sds s, char *t, size_t len);
 sds sdscpy(sds s, char *t);
-sds sdssetbit(sds s, size_t bit, int on);
 
 sds sdscatvprintf(sds s, const char *fmt, va_list ap);
 #ifdef __GNUC__