Shared: Add a Universal Flow library and refactor TypeFlow to use it.

[aschackmull]

Universal flow is the dual of existential flow in the sense that universal quantification replaces existential quantification in the recursive step in the reachability calculation. The existing shared TypeFlow library uses universal flow in several instances to prove additional type bounds that are stricter than the given static type.

This PR extracts the core universal flow calculation from TypeFlow and puts it in a separate library, such that we can easily apply it to other use-cases beyond TypeFlow. In order to do this, I had to make some very minor tweaks to the TypeFlow predicates, so the calculated bounds are not exactly the same, but they're very close:

• exactType should be preserved tuple-for-tuple.
• typeFlow now includes exactType as additional bounds directly instead of accounting for these in the (prior) supportsType predicate. This simplifies things and only makes a tiny difference since the exact bounds are filtered afterwards.
• hasUnionTypeFlow shifts slightly to put the base case on nodes rather than both nodes and edges. Again this simplifies things and only makes a tiny difference in the end when superfluous bound disjuncts are pruned.

Commit-by-commit review is encouraged - I've tried hard to break the refactoring down into easily reviewable parts.

[aschackmull]

The second commit TypeFlow: Simplify interface caused a slight semantic change which is now rectified in UniversalFlow: Skip property propagation to null nodes.

[hvitved]

Looks plausible to me.