Compare public fields in execution-results.h

[tlively]

When comparing two logged struct or array values, look up their closest public supertypes. If these supertypes are different, then the values are observably different. Otherwise, look at the fields the shared public supertype makes visible and recursively check for differences. Since GC data may in general be recursive, naively recursing may cause the comparison to exhaust the stack or fail to terminate. Instead, perform a coinductive equality check where pairs of values are assumed to be equivalent until proven otherwise. This avoids recursing into the comparison of a pair of values we have already started comparing.

Since two execution traces typically differ only when the fuzzer finds a misoptimization, add a new gtest file to test the new comparison logic.

[tlively]

There is a known issue with this where unsubtyping makes makes it so a private subtype of a public type is no longer a subtype of that public type, which causes the comparison to fail. It seems we need to record the static type with which each particular value was exposed rather than using public supertypes.