Improve deepEqual() by changing iteration order

[creasty]

Code change checklist

• [-] Added/updated unit tests
• [-] Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

Why

A rational engineer would define significant properties (which are typically distinctive, such as IDs) at the beginning of an object when modeling.
The current logic searches properties in reverse order of addition, which could be inefficient in many cases.

cf. https://2ality.com/2015/10/property-traversal-order-es6.html

[changeset-bot]

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