Add sdsnative()

Use the existing memory space for an SDS to convert it to a regular
character buffer so we don't need to allocate duplicate space just
to extract a usable buffer for native operations.

src/sds.h

@@ -60,6 +60,7 @@ sds sdsempty(void);
 size_t sdslen(const sds s);
 sds sdsdup(const sds s);
 void sdsfree(sds s);
+char *sdsnative(sds s);
 size_t sdsavail(const sds s);
 sds sdsgrowzero(sds s, size_t len);
 sds sdscatlen(sds s, const void *t, size_t len);