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`