implement bitfield signing subsystem (#1364)

* 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 <[email protected]>

* 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 <[email protected]>