Add 'mpls()' function for creating MPLS header with parameters:
lbl|label MPLS label
lastIndicates the last label on MPLS stack
tc|tclass|exp Traffic Class (TC)
ttl TTL (Time To Live)
Currently only unicast MPLS is supported, but
Current logic does OR of existing field value & shift-masked
specified value, which is not enough as 0s bits from
the specified value will be not set (be cause of OR),
so fixed it by reseting original field value by AND with
reverted field mask, in otherwords - bits part of original field
value is
