Minimal collation work-flow and necessary traits (#25)

* collator crate skeleton and description

* parachain primitives: proof -> witness and egress format

* collation of ingress queues through trait

* add ingress collation test

* structure for collated ingress

* add collated ingress to proposal

* witness -> proof

* ingress collation and candidate creation + code cleanup

* update collator lib to new definitions

* address formatting grumble