Using Exchange doesn't add anything, as it just wraps client.Exchange
with a default client.

Remove them and speed up the tests, goes from 3s to 1s (for the entire
test suite).

Signed-off-by: Miek Gieben <miek@miek.nl>

This adds hash.go and creates a identityHash that is used for algorithms
that do their own hashing (ED25519) for instance.

This unifies the hash variable naming between dnssec and sig(0) signing
and removes the special casing that existed for ED25519.

This unifies the variable naming between sig(0) and dnssec signing and
verifying.

I didn't want to used crypto.RegisterHash as not to fiddle with the
global namespaces of hashes, so the value of '0' from AlgorithmsToHash
is handled specially in dnssec and sig(0) code.

Note that ED448 isn't implemented at all.

Signed-off-by: Miek Gieben <miek@miek.nl>

) (#1293)

* Change EDNS_EXPIRE field to support zero length option data (Resolves #1292)

As per [RFC7134](https://datatracker.ietf.org/doc/html/rfc7314#section-2) the Expire
Option in queries should be zero-length. In the current implementation the field is
uint32 which always instatiates 4bytes for that field when packing to wire format.
For that reason we change the field to []uint8 so it can support 0-length and 4-byte
length option data.

* addressed comments

* addressed comments

* make change backwards compatible

* add comment for Empty field