struct secret: use everywhere.

We alternated between using a sha256 and using a privkey, but there are
numerous places where we have a random 32 bytes which are neither.

This fixes many of them (plus, struct privkey is now defined in terms of
struct secret).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

type_to_string.h

@@ -21,6 +21,7 @@ union printable_types {
 	const secp256k1_pubkey *secp256k1_pubkey;
 	const struct channel_id *channel_id;
 	const struct short_channel_id *short_channel_id;
+	const struct secret *secret;
 	const struct privkey *privkey;
 	const secp256k1_ecdsa_signature *secp256k1_ecdsa_signature;
 	const struct channel *channel;