Use Base.IdDict instead of objectid to store subexpressions

[odow]

Closes #4203

I didn't realise objectid could be re-used, but I guess it makes sense. It's not documented very well.

But this is exactly what I didn't want to happen, because we don't want to keep every single subexpression alive for the life of the model. That could be a big increase in memory usage.

One option is to make the subexpression storage opt-in.

[odow]

I would have much preferred that we just exploited common subexpressions within a single call to moi_function: #4032 (comment)

Then there's no model-level state that we are tracking.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (62b9c47) to head (22f4ae5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4204      +/-   ##
==========================================
+ Coverage   99.95%   99.96%   +0.01%     
==========================================
  Files          43       43              
  Lines        6306     6313       +7     
==========================================
+ Hits         6303     6311       +8     
+ Misses          3        2       -1     

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[AayushSabharwal]

To avoid keeping expressions alive, something like a WeakKeyDict is useful. It uses WeakRef to store the keys, allowing the GC to collect them, and a finalizer to remove it from the dictionary when they get cleaned up. However, the implementation in Base is pretty slow iirc. SymbolicUtils uses WeakCacheSets.jl for hashconsing. It's a set not a dict, so it isn't directly applicable here, but it illustrates the sort of approach that we found to work well in such scenarios.

[odow]

I need to:

• Benchmark the current approach
• Benchmark something based on https://docs.julialang.org/en/v1/base/base/#Core.WeakRef
• Consider making it opt-in via @expression
• Consider a global flag to turn off subexpression caching

[odow]

Closing because I don't like this. At minimum, we can do something like #4207. Or maybe WeakRef + @expression.