Heap2Local: Track interactions in detail

[kripken]

Previously we tracked only whether an expression was relevant to analysis, that is,
whether it interacted with the allocation we were tracing the behavior of. That is
not enough for all cases, though, so also track the form of the interaction, namely
whether the allocation flows through or is fully consumed. An example where that
matters:

(ref.eq
  (struct.get $A 0
    (local.tee $x
      (struct.new_default $A)
    )
  )
  (local.get $x)
)

Here the local.get flows out the allocation, but the struct.get only fully consumes
it. Before this PR we thought the struct.get flowed the allocation, and we misoptimized
this to 1.

To make this possible, do a bunch of minor refactoring:

• Move ParentChildInteraction out of the class.
• Add a "None" interaction there.
• Replace the set of reached expressions with a map of them to their interactions.
• Add helper functions to get an expression's interaction or to update it when replacing.

The new testcase here shows the main fix. The new assertions are covered by existing
testcases.

[tlively]

Was this found by the fuzzer?

[kripken]

Yes, fuzzer.