Add common/Leaf struct & parsers (bigint & byte arrays)

common/utils.go

@@ -0,0 +1,10 @@
+package common
+
+// SwapEndianness swaps the order of the bytes in the slice.
+func SwapEndianness(b []byte) []byte {
+	o := make([]byte, len(b))
+	for i := range b {
+		o[len(b)-1-i] = b[i]
+	}
+	return o
+}