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/commonTest/kotlin/bipack/BipackEncoderTest.kt

@@ -11,11 +11,11 @@ import kotlin.test.*
 data class Foobar1N(val bar: Int, val foo: Int = 117)
 
 @Serializable
-@ExtendableFormat
+@Extendable
 data class Foobar1(val bar: Int, val foo: Int = 117)
 
 @Serializable
-@ExtendableFormat
+@Extendable
 @SerialName("bipack.Foobar1")
 data class Foobar2(val bar: Int, val foo: Int, val other: Int = -1)
 
@@ -30,12 +30,12 @@ data class FoobarF2(val bar: Int, val foo: Int, val other: Int = -1)
 
 @Serializable
 @Framed
-@ExtendableFormat
+@Extendable
 data class FoobarF3(val bar: Int, val foo: Int, val other: Int = -1)
 
 @Serializable
 @Framed
-@ExtendableFormat
+@Extendable
 @CrcProtected()
 data class FoobarFP1(val bar: Int, val foo: Int, val other: Int = -1)