Instances of ArithmeticObject on GS64 are now invariant

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.93%. Comparing base (41a3c15) to head (8469eca).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-candidate #34 +/- ## ================================================== Coverage 93.93% 93.93% ================================================== Files 57 57 Lines 10061 10062 +1 ================================================== + Hits 9451 9452 +1 Misses 610 610 ```

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[ytsejam78]

Tested with:

UserGlobals at: #InvariantMeasureTest put: ((BaseUnit named: 'metro' sign: 'm') with: 1).
System commitTransaction.
(UserGlobals at: #InvariantMeasureTest) instVarAt: 1 put: 2.

raises error: "a ArgumentTypeError occurred (error 2031), Attempt to modify invariant object, aMeasure"