feerate: add a 'common_weight' calculation for a tx

The bits of a transaction that are paid by the opener!

bitcoin/feerate.c

@@ -1,4 +1,5 @@
 #include <bitcoin/feerate.h>
+#include <bitcoin/varint.h>
 #include <limits.h>
 #include <stdlib.h>
 
@@ -38,3 +39,12 @@ const char *feerate_style_name(enum feerate_style style)
 	}
 	abort();
 }
+
+size_t common_weight(size_t num_inputs, size_t num_outputs)
+{
+	/*(nVersion + num inputs + num outputs + locktime) * 4
+	 *         + SegWit marker + SegWit flag */
+	return (4 + varint_size(num_inputs) +
+		varint_size(num_outputs) + 4) * 4
+		+ 1 + 1;
+}

bitcoin/feerate.h

@@ -34,6 +34,11 @@
  */
 #define FEERATE_FLOOR 253
 
+/*
+ * This is the net common weight of a transaction.
+ */
+size_t common_weight(size_t num_inputs, size_t num_outputs);
+
 enum feerate_style {
 	FEERATE_PER_KSIPA,
 	FEERATE_PER_KBYTE