Shared runtime

This already bitrots for about a year and it's still very big.
Piecewise merging caused delays for several months and
I also lack an idea how to further split it down.
I thought we might try something different to make this reviewable.

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This serves as __Meta Pull Request__, please __don't pull it__.
I'll create subsequent pull requests for each commit
which should get discussed and merged separately.
I'll regularly rebase this one until hopefully nothing is left.