Context links, new pipelining syntax

[tbarbette]

MiddleClick (HSPR '18 + Tom Barbette thesis) proposes the context link. A link, noted as ~>, allows to automatically introduce a classifier that uses the element on the right hand side of the arrow to discover a rule. It would be great to have it in a more general way (not tied to the MiddleClick subsystem) in FastClick. E.g., one could have :
fd :: FromDevice;
fd ~> [1]ARPQuerier
fd ~> ARPResponder
fd ~> CheckIPHeader
Without the old same Classifier(). This is a streamlined default case.

[tbarbette]

From @gkatsikas :
How can you infer the different rules of the Classifier? Do you map elements to their header offsets? E.g., in the example you provided above a CheckIPHeader corresponds to a 12/800 rule offset.

[tbarbette]

The element declares a rule proper to its own protocol and a "parent type", ARPQuerier says "20/0001", type FLOW_ARP. Then when resolving the NIC, the previous "flow" element is asked how to resolve FLOW_ARP, that would return "12/0806", and the child is then appended. The parent type allows supporting encapsulation. Eg in IP over something you don't want to add 12/0800.