Frozen tables should use reference equality

[dphfox]

For a while there have been questions about how best to deal with table mutations, since they break Fusion's assumptions about how equality works. Right now, Fusion is pessimistic; it treats tables as always dissimilar, even to themselves, to ensure we don't skip over changes.

However, if a table is frozen, this mutability becomes less of an issue. The only mutation to be wary of is mutation inside of nested tables, but I think it's a safe assumption that developers will treat frozen tables as immutable and constant, and prefer to use state objects to model any interior mutability.

For those reasons, I think we can reintroduce table reference equality, but only for frozen tables.