optimization: ise varint on field counter for extendable format (it's rare for a struct to have so many fields so smartint will be better).

src/commonMain/kotlin/net.sergeych.bintools/DataSink.kt

@@ -22,6 +22,11 @@ interface DataSink {
     fun writeBytes(data: ByteArray) {
         for(d in data) writeByte(d)
     }
+
+    fun writeVarUInt(value: UInt) { Varint.encodeUnsigned(value.toULong())}
+    fun writeVarInt(value: UInt) { Varint.encodeSigned(value.toLong())}
+    fun writeSmartUInt(value: UInt) { Smartint.encodeUnsigned(value.toULong())}
+    fun writeSmartInt(value: UInt) { Smartint.encodeSigned(value.toLong())}
 }
 
 inline fun <reified T:Any>DataSink.writeNumber(value: T) {