Reland "[dataflow] Add dedicated representation of boolean formulas"

This reverts commit 7a72ce98.

Test problems were due to unspecified order of function arg evaluation.

Reland "[dataflow] Replace most BoolValue subclasses with references to Formula (and AtomicBoolValue => Atom and BoolValue => Formula where appropriate)"

This properly frees the Value hierarchy from managing boolean formulas.

We still distinguish AtomicBoolValue; this type is used in client code.
However we expect to convert such uses to BoolValue (where the
distinction is not needed) or Atom (where atomic identity is intended),
and then fold AtomicBoolValue into FormulaBoolValue.

We also distinguish TopBoolValue; this has distinct rules for
widen/join/equivalence, and top-ness is not represented in Formula.
It'd be nice to find a cleaner representation (e.g. the absence of a
formula), but no immediate plans.

For now, BoolValues with the same Formula are deduplicated. This doesn't
seem desirable, as ...