common: Add multiplication primitives for amount_msat and amount_sat

The scale variants weren't usable since they use `double` as a scale
factor, which in turn causes imprecisions.

common/amount.c

@@ -507,6 +507,22 @@ struct amount_sat amount_sat_div(struct amount_sat sat, u64 div)
 	return sat;
 }
 
+bool amount_sat_mul(struct amount_sat *res, struct amount_sat sat, u64 mul)
+{
+	if (	mul_overflows_u64(sat.satoshis, mul))
+		return false;
+	res->satoshis = sat.satoshis * mul;
+	return true;
+}
+
+bool amount_msat_mul(struct amount_msat *res, struct amount_msat msat, u64 mul)
+{
+	if (	mul_overflows_u64(msat.millisatoshis, mul))
+		return false;
+	res->millisatoshis = msat.millisatoshis * mul;
+	return true;
+}
+
 bool amount_msat_fee(struct amount_msat *fee,
 		     struct amount_msat amt,
 		     u32 fee_base_msat,

common/amount.h

@@ -92,6 +92,9 @@ WARN_UNUSED_RESULT bool amount_sat_scale(struct amount_sat *val,
 struct amount_msat amount_msat_div(struct amount_msat msat, u64 div);
 struct amount_sat amount_sat_div(struct amount_sat sat, u64 div);
 
+bool amount_sat_mul(struct amount_sat *res, struct amount_sat sat, u64 mul);
+bool amount_msat_mul(struct amount_msat *res, struct amount_msat msat, u64 mul);
+
 /* Is a == b? */
 bool amount_sat_eq(struct amount_sat a, struct amount_sat b);
 bool amount_msat_eq(struct amount_msat a, struct amount_msat b);