script: Use pkh to construct p2pkh output scripts

So far we always needed to know the public key, which was not the case
for addresses that we don't own. Moving the hashing outside of the
script construction allows us to send to arbitrary addresses. I also
added the hash computation to the pubkey primitives.

bitcoin/pubkey.c

@@ -99,3 +99,13 @@ static char *privkey_to_hexstr(const tal_t *ctx, const struct privkey *secret)
 }
 REGISTER_TYPE_TO_STRING(privkey, privkey_to_hexstr);
 REGISTER_TYPE_TO_HEXSTR(secret);
+
+void pubkey_to_hash160(const struct pubkey *pk, struct ripemd160 *hash)
+{
+	u8 der[PUBKEY_DER_LEN];
+	struct sha256 h;
+
+	pubkey_to_der(der, pk);
+	sha256(&h, der, sizeof(der));
+	ripemd160(hash, h.u.u8, sizeof(h));
+}