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.h

@@ -1,6 +1,8 @@
 #ifndef LIGHTNING_BITCOIN_PUBKEY_H
 #define LIGHTNING_BITCOIN_PUBKEY_H
 #include "config.h"
+#include <ccan/crypto/ripemd160/ripemd160.h>
+#include <ccan/crypto/sha256/sha256.h>
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/tal.h>
 #include <secp256k1.h>
@@ -38,4 +40,9 @@ bool pubkey_eq(const struct pubkey *a, const struct pubkey *b);
 
 /* Compare the keys `a` and `b`. Return <0 if `a`<`b`, 0 if equal and >0 otherwise */
 int pubkey_cmp(const struct pubkey *a, const struct pubkey *b);
+
+/**
+ * pubkey_to_hash160 - Get the hash for p2pkh payments for a given pubkey
+ */
+void pubkey_to_hash160(const struct pubkey *pk, struct ripemd160 *hash);
 #endif /* LIGHTNING_PUBKEY_H */