cryptobyte: add ReadUint64 and AddUint64

Fixes golang/go#53481.

Change-Id: Ic00eef498d1d3b5b0ca5c9c526fac7c26de30cf2
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/421014
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Run-TryBot: hopehook <hopehook@qq.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

cryptobyte/builder.go

@@ -95,6 +95,11 @@ func (b *Builder) AddUint32(v uint32) {
 	b.add(byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
 }
 
+// AddUint64 appends a big-endian, 64-bit value to the byte string.
+func (b *Builder) AddUint64(v uint64) {
+	b.add(byte(v>>56), byte(v>>48), byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
+}
+
 // AddBytes appends a sequence of bytes to the byte string.
 func (b *Builder) AddBytes(v []byte) {
 	b.add(v...)