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/string.go

@@ -81,6 +81,17 @@ func (s *String) ReadUint32(out *uint32) bool {
 	return true
 }
 
+// ReadUint64 decodes a big-endian, 64-bit value into out and advances over it.
+// It reports whether the read was successful.
+func (s *String) ReadUint64(out *uint64) bool {
+	v := s.read(8)
+	if v == nil {
+		return false
+	}
+	*out = uint64(v[0])<<56 | uint64(v[1])<<48 | uint64(v[2])<<40 | uint64(v[3])<<32 | uint64(v[4])<<24 | uint64(v[5])<<16 | uint64(v[6])<<8 | uint64(v[7])
+	return true
+}
+
 func (s *String) readUnsigned(out *uint32, length int) bool {
 	v := s.read(length)
 	if v == nil {