Switch ASN1_item_sign_ctx() to EVP_DigestSign()

Switch ASN1_item_sign_ctx() to EVP_DigestSign()

This makes this function work with Ed25519 and cleans up a handful of
ugly contortions: use EVP_DigestSign() to determine the signature length
instead of using the strange EVP_PKEY_size() and garbage collect the now
useless out_len. Also use calloc().

ok jsing