archive/tar: Make Reader and Writer more closely follow io.Reader and io.Writer interfaces

There's no functional change here. io gives the Read and Write methods byte slice arguments, but tar called them uint8. It's the same thing, but I think this is clearer and it matches what other packages do.

R=rsc
CC=golang-dev
https://golang.org/cl/157099

src/pkg/archive/tar/reader.go

@@ -204,7 +204,7 @@ func (tr *Reader) readHeader() *Header {
 // Read reads from the current entry in the tar archive.
 // It returns 0, nil when it reaches the end of that entry,
 // until Next is called to advance to the next entry.
-func (tr *Reader) Read(b []uint8) (n int, err os.Error) {
+func (tr *Reader) Read(b []byte) (n int, err os.Error) {
 	if int64(len(b)) > tr.nb {
 		b = b[0:tr.nb]
 	}

src/pkg/archive/tar/writer.go

@@ -163,7 +163,7 @@ func (tw *Writer) WriteHeader(hdr *Header) os.Error {
 // Write writes to the current entry in the tar archive.
 // Write returns the error ErrWriteTooLong if more than
 // hdr.Size bytes are written after WriteHeader.
-func (tw *Writer) Write(b []uint8) (n int, err os.Error) {
+func (tw *Writer) Write(b []byte) (n int, err os.Error) {
 	overwrite := false;
 	if int64(len(b)) > tw.nb {
 		b = b[0:tw.nb];