ecc: Use opaque bytes by MPI (was: An octet sequence as an MPI)

[Niibe Yutaka]

Hello,

In master, I clean up the code of ECC.  Now, for point representation
(in public key, in secret key, and in ephemeral key), it uses an opaque
bytes internally.

We have public API:

    gcry_mpi_ec_get_mpi
    gcry_mpi_ec_set_mpi

which is questionable these days (it's useful when we do experiment on
ECC domain parameters).  I don't touch here.

For the public API gcry_mpi_ec_decode_point, it supports both
cases of an opaque bytes and normal MPI.

I found that for EdDSA, while the 0x40 prefix is used in public key
and secret key, in r of signature, there is no prefix.  The places
where we still have problems are:

   EdDSA r of signature: It's native point representation in little
   endian with no prefix.  Zero-byte at the beginnings may be removed,
   thus we have left-pad-zero handling for input.  For input, no prefix
   is allowed.

   EdDSA s of signature: It's native little endian.  Zero-byte at the
   beginnings may be removed, thus we have left-pad-zero handling for
   input.

   EdDSA private key bytes: It's fixed-length bytes.  Zero-byte at the
   beginnings may be removed, thus we have left-pad-zero handling, for
   input.

I don't change those, for now.
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