impl ProvideInherent for InclusionInherent (#1318)

* impl ProvideInherent for InclusionInherent

* reduce import churn; correct expect message

* move inclusion inherent identifier into primitives

It's not clear precisely why this is desired, but it's a pattern
I've seen in several places, so I'm going this to be on the
safe side. Worst case, we can revert this commit pretty easily.

* bump kusama spec_version to placate CI

* add license header

* empty commit; maybe github will notice the one with changes

* add sanity check to only include valid inherents
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