MultisetSubstitutionSystem: allow patterns like {a__, b__} to be matched multiple times

[maxitg]

Changes

• Closes #637.
• Left-hand sides of rules such as {a__, b__} will now be matched multiple times even if the ordered sequence of tokens is the same (e.g., as {{a} -> {1}, {b} -> {2, 3}} and {{a} -> {1, 2}, {b} -> {3}}).
• This was done by allowing the same sequence of tokens to be matched multiple times (how many times is computed during the matching step).
• This required adding another ordering function, "InstantiationIndex", which controls which of the instantiations will be used first. (There is still no customization of the ordering at the moment.)

Examples

• The example from the issue can now generate both matches:

In[] := #["ExpressionsEventsGraph", VertexLabels -> Automatic] &@
 SetReplaceTypeConvert[{WolframModelEvolutionObject, 2}]@
  GenerateMultihistory[
    MultisetSubstitutionSystem[{a__, b__} /; OrderedQ[{a, b}] :> {{a}, {b}}], {"MaxGeneration" -> 1,
     "MinEventInputs" -> 3, "MaxEventInputs" -> 3},
    None, {"InputCount", "SortedInputTokenIndices", "InputTokenIndices", "RuleIndex"}, {}][{1, 2, 3}]

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This change is 

[maxitg]

I fixed the memory leak, so it's ready for review now. I ended up using @taliesinb's suggestion about Data`UnorderedAssociation from a previous PR after all. It turned out, not only it has a better API, but it is more reliable as well.