varint: Add helper function for getting varlen size

Needed to calculate somethings for building dual funding txs

bitcoin/varint.c

@@ -1,5 +1,16 @@
 #include "varint.h"
 
+size_t varint_size(varint_t v)
+{
+	if (v < 0xfd)
+		return 1;
+	if (v <= 0xffff)
+		return 3;
+	if (v <= 0xffffffff)
+		return 5;
+	return 9;
+}
+
 size_t varint_put(u8 buf[VARINT_MAX_LEN], varint_t v)
 {
 	u8 *p = buf;

bitcoin/varint.h

@@ -9,6 +9,9 @@
 
 #define VARINT_MAX_LEN 9
 
+/* Calculate bytes used (up to 9) */
+size_t varint_size(varint_t v);
+
 /* Returns bytes used (up to 9) */
 size_t varint_put(u8 buf[VARINT_MAX_LEN], varint_t v);