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

ipv4/icmpv4/icmpv4.go

@@ -3,8 +3,6 @@ package icmpv4
 import (
 	"encoding/binary"
 	"errors"
-
-	"github.com/soypat/lneto"
 )
 
 type Type uint8
@@ -87,12 +85,6 @@ func (frm Frame) SetCRC(crc uint16) {
 	binary.BigEndian.PutUint16(frm.buf[2:4], crc)
 }
 
-// CRCWrite calculates the checksum of the ICMP packet. Treats the checksum field as zero as per RFC 792.
-func (frm Frame) CRCWrite(crc *lneto.CRC791) {
-	crc.AddUint16(binary.BigEndian.Uint16(frm.buf[0:2]))
-	crc.Write(frm.buf[4:])
-}
-
 func (frm Frame) payload() []byte {
 	return frm.buf[4:]
 }