bytes, strings: optimize Repeat

Call copy with as large buffer as possible to reduce the
number of function calls.

benchmark                 old ns/op    new ns/op    delta
BenchmarkBytesRepeat            540          162  -70.00%
BenchmarkStringsRepeat          563          177  -68.56%

LGTM=josharian
R=golang-codereviews, josharian, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/90550043

src/pkg/bytes/bytes_test.go

@@ -1232,3 +1232,9 @@ func BenchmarkTrimSpace(b *testing.B) {
 		TrimSpace(s)
 	}
 }
+
+func BenchmarkRepeat(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Repeat([]byte("-"), 80)
+	}
+}