pick off special one-byte case in copy. worth 2x in benchmarks (38ns->16ns).

the one-item case could be generalized easily with no cost. worth considering.

R=rsc
CC=golang-dev, cw
https://golang.org/cl/167044

src/pkg/runtime/slice.c

@@ -208,7 +208,11 @@ runtime·slicecopy(Slice to, Slice fm, uintptr width, int32 ret)
 	if(to.len < ret)
 		ret = to.len;
 
-	memmove(to.array, fm.array, ret*width);
+	if(ret == 1 && width == 1) {	// common case worth about 2x to do here
+		*to.array = *fm.array;	// known to be a byte pointer
+	} else {
+		memmove(to.array, fm.array, ret*width);
+	}
 
 out:
 	FLUSH(&ret);