Checksum simplification proposal (#29)

* Simplified checksum calculation and verification

* Fixed tests

* UDP: using NewBoundedFrame to create a Frame instance limited to the actual frame size.

* Minor: renamed some functions

* Commented and made more readable consecutive SetCRC calls.

* Fixed icmp checksum calculation after rebase

* Replaced NewBoundedFrame with explicit validation

x/xnet/stack-async.go

@@ -567,6 +567,5 @@ func addr4(addr [4]byte, ok bool) netip.Addr {
 
 func hash(b []byte) uint16 {
 	var csum lneto.CRC791
-	csum.Write(b)
-	return csum.Sum16()
+	return csum.PayloadSum16(b)
 }