Add runtime support for `PreRuntime` and `Consensus` digests (#2757)

* Try to fix runtime panic

Does not work

* Fix trivial typo

* Add runtime support for `PreRuntime` and `Consensus` digests

Fixes 6e253c89



* Fix silly compile error.

* Fix overly-long lines

Also remove some in-progress code that would not wind up being useful
anyway.

* Respond to review comments

* delete `unset RUSTC_WRAPPER` from scripts/common.sh
* delete unnecessary `use aura::AURA_ENGINE_ID` from
  `node/runtime/src/lib.rs`
* add comments explaining why `PreRuntime` and `Consensus` must be
  special-cased in `core/sr-primitives/lib.rs`
* switch to using `$crate::rstd::marker::PhantomData` in
  `impl_outer_log!`
* improve documentation of `DigestItem::Seal`

* Fix compilation and add proof that we do not panic

Also fix some warnings.

* Apply suggestions from code review

Mostly for readability

Co-Authored-By: Sergei Pepyakin <[email protected]>

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* $crate::rstd::marker::PhantomData → Default::default()

The import is still needed, as `Default::default()` can’t be used in
patterns.

* Bump `spec_version`

Also do some reformatting.