Inclusion Module (#1242)

* add availability bitfield types to primitives

* begin inclusion module

* use GitHub issue link for limitation

* fix some compiler errors

* integrate validators into initializer

* add generic signing context

* make signing-context more generic

* fix issues with inclusion module

* add TODO

* guide: add validators and session index to inclusion

* guide: add session index to change notification

* implement session change logic

* add BackedCandidate type

* guide: refine inclusion pipeline

* guide: rename group_on to group_validators

* guide: add check about collator for parathread

* guide: add last_code_upgrade to paras and use in inclusion

* implement Paras::last_code_upgrade

* implement most checks in process_candidates

* make candidate receipt structs more generic

* make BackedCandidate struct more generic

* use hash param, not block number

* check that candidate is in context of the parent block

* include inclusion module in initializer

* implement enact-candidate

* check that only occupied cores have bits set

* finish implementing bitfield processing

* restructure consistency checks on candidates

* make some more primitives generic

* signature checking logic for backed candidates

* finish implementing process_candidates

* implement collect_pending

* add some trait implementations to primitives

* implement InclusionInherent and squash warnings

* test bitfield signing checks

* rename parachain head to para_head

* fix note_new_head bug in paras

* test bitfield enactment in inclusion

* helpers for candidate checks

* add test for most candidate checks

* add test for backing setting storage

* test session change logic

* remove extraneous type parameter

* remove some allow(unused)s

* extract threshold computation to const fn

* remove some more allow(unused)s

* improve doc

* add debug assertion

* fix primitive test compilation

* tag unanimous variant as unused