cv25519 scalar byte order

[NIIBE Yutaka]

Hello,

Here is an example packet of secret key as an output of GnuPG's
--list-packets.

==========================
# off=267 ctb=9c tag=7 hlen=2 plen=93
:secret sub key packet:
	version 4, algo 18, created 1518583322, expires 0
	pkey[0]: 0A2B060104019755010501 cv25519 (1.3.6.1.4.1.3029.1.5.1)
	pkey[1]: 40FD57ED7E6D3490F0BFB4995F281A53648842CDB84DEBA6726DF5BA1994968D23
	pkey[2]: 03010807
	skey[3]: 5669627AB7427CE223BD94101603978F3F4599AE1A007087A0B3DBBFB67D3278
	checksum: 0f5a
	keyid: 55C6A075A3C82F68
==========================

I wonder if we have difference in the interpretation of secret part
(skey[3]).

In GnuPG, this part is interpreted as standard MPI representation
(big-endian).

For better interoperability, we could support the prefix 0x40 for this
secret part, I suppose.
--