* some fixes to please cargo-spellcheck

* some (not all) fixes for the impl guide

* fix

* guide: reversion safety

* guide: manage reversion safety in subsystems

* add leaf status to ActivatedLeaf

* add an LRU-cache to overseer for staleness detection

* update ActivatedLeaf usages in tests to contain status field

* add variant where missed accidentally

* add some helpers to LeafStatus

* address grumbles

* guide: add candidate information to OccupiedCore

* add descriptor and hash to occupied core type

* guide: add candidate hash to inclusion

* runtime: return candidate info in core state

* bitfield signing: stop querying runtime as much

* minimize going to runtime in availability distribution

* fix availability distribution tests

* guide: remove para ID from Occupied core

* get all crates compiling

* update guide to reduce confusion and TODOs

* work from previous bitfield signing effort

There were large merge issues with the old bitfield signing PR, so
we're just copying all the work from that onto this and restarting.

Much of the existing work will be discarded because we now have better
tools available, but that's fine.

* start rewriting bitfield signing in terms of the util module

* implement construct_availability_bitvec

It's not an ideal implementation--we can make it much more concurrent--
but at least it compiles.

* implement the unimplemented portions of bitfield signing

* get core availability concurrently, not sequentially

* use sp-std instead of std for a parachain item

* resolve type inference failure caused by multiple From impls

* handle bitfield signing subsystem & Allmessages variant in overseer

* fix more multi-From inference issues

* more concisely handle overflow

Co-authored-by: Andronik Ordian <write@reusable.software>

* Revert "resolve type inference failure caused by multiple From impls"

This reverts commit 7fc77805.

* Revert "fix more multi-From inference issues"

This reverts commit f14ffe58

.

* impl From<i32> for ParaId

* handle another instance of AllSubsystems

* improve consistency when returning existing options

Co-authored-by: Andronik Ordian <write@reusable.software>

* rename implementor's guide to implementer's guide

* fix typos in more places

* Add mdbook-linkcheck

* Convert todo to not a link

* Fix external glossary link

* Fix various broken links

* Mass replace: s/.html/.md/

* My sloppy attempt to add a gitlab CI check

* adjust to relative links

* additional adjustment to relative link

* link directly to file

* link directly to file

* move old implementers' guide, add skeleton of new

* Split the old implementers' guide into the new one's sections

This is mostly a straightforward copying operation, moving the
appropriate sections from the old guide to the new. However, there
are certain differences between the old text and the new:

- removed horizontal rules between the sections
- promoted headers appropriately within each section
- deleted certain sections which were in the old guide's ToC but
  which were not actually present in the old guide.
- added Peer Set Manager to the new ToC

* remove description headers

It is redundant and unnecessary. Descriptions fall directly under the
top-level header for any given section.

* add stub description of the backing module

* add stub description for the availability module

* add stub description for collators

* add stub description for validity

* add stub description for utility

* highlight TODO and REVIEW comments

* add guide readme describing how to use mdbook

* fix markdownlint lints

* re-title parachains overview

* internal linking for types

* module and subsystem internal links

* .gitignore should have a trailing newline

* node does not have modules, just subsystems